to help me understand friend with chronic fatigue

You don't want to understand her. You want her to stop talking about her condition and give you support. It doesn't sound as though that is going to happen.
She feels because she has a chronic condition that she's at the centre of the support circle. That means she moans to you, and you should moan out to the next layer of the circle.
Your OP has lots of mean-spirited undertones and she might be picking up on them in real life eg she goes out, she has holidays,etc. Either way, it doesn't sound like this friendship is working for either of you.

And I don't want to presume anything about her, but if she is getting out and about now and again, and is able to meet up with you from time to time then I'm sure she must have the energy to not talk about herself for half an hour and just focus on you.

Yes, it can make you self focused and a bit selfish at times, but anyone with a bit of insight can see when they need to step outside themselves and focus on a friend for a while.

I have CFS, amongst other disabilities, and so I understand how she feels with how overwhelming it feels, like nothing else matters much because it’s all you can think about every second, of every day. Nothing is ever straightforward with it, and it’s linked to most (almost all probably) decisions I make, whether that’s to go to a friends birthday, or to have my hair cut so it’s easier to cope with etc. I can see how it can become all encompassing in terms of conversation, altho I find I go the other way, I consciously try not to mention it too much in case it bores people, or they judge me, or think ‘it’s not that bad’ if I go out.
I can understand your frustration at this being the only topic of conversation however. As a pp says, if you talk of entirely unrelated stuff, what happens then? I do think part of being a friend is listening to the others struggles but definitely not all the time.

She’s probably having to rest a lot in between times, she may then struggle to find the energy for basic things.

I do think she should be able to listen to you as well and not make everything about herself.

I have ME and I don't harp on about it all the time. Yes, it's shit. And it's different for everyone. No doubt it's horrendous for her, but yes, you have your difficulties too. It must be tiring for her to harp on about her illness so much!

If you're doing your best to be an understanding friend and she's not returning the favour (even occasionally) then it's probably best if you withdraw. It's not fair on you.

And- I know some things are not 'over', like chronic illness, but that's why you have to pace those around you and not moan constantly! Or find another outlet for your upset.

The proportion of CFs (cheeky fuckers) with CFS will mirror the proportion of CFs in the general population. Having CFS doesn't absolve you of being a CF.

I don't think it's reasonable to be a friend who is boring and self-obsessed. It doesn't matter what it's about- illness, your children, your health, your lifestyle, all friendships work better if there's mutual interest and understanding, and that yes, sometimes one thing will dominate for a while, but then that will be over and you can be mutually supporting. It sounds like you have a tough life too, and so ideally she could support you and vice versa, but that's not happening. I do know three people with CFS, one mentions it occasionally if relevant, one is very upbeat but naturally limits what they do energy wise and one is very downbeat and draining and has lost a lot of friends over the past decades due to their self-obsession. People do have a choice. I also think that it helps if you have a place where others will understand easily which is not the case always with CFS and the ones that do better tend to be in support groups/online chats as well as have real life friends as that's a place to put all that discussion where perhaps others (who don't have CFS) might not be as sympathetic or interested in the detail of new treatments. Can you encourage her to reach out to others with the same issue?

What's she like if you talk about other subjects altogether, eg: Trump, the weather, EastEnders, some new jewellery, etc etc?

It is an illness that makes you introverted and quite self obsessed - as so many debilitating illnesses do. If you are prevented from living a full life, then the main topic of conversation is your illness.

But I think you could have a word. Explain that you think it would be good for her to talk about other things and give herself a break from thinking about it all the time.

If she won't then inevitably your time with her will reduce. If she has the energy to talk about herself, then she has the energy to talk about something that concerns or interests you for part of the time too. And I say that as someone who suffers from depression that creates chronic fatigue-like symptoms.

You said she has a pretty full on social life.

I think that whatever your struggles you don’t get to just dismiss other people’s and check out of being a friend.

I’m really struggling with pain and fatigue at the moment. I’ve barely seen my friends because I need all my energy for work and it can be hard to listen to their life dramas when at times it feels trivial.

But it’s not their fault I feel rubbish and actually they have their own problems to deal with and yes it is exciting to have new carpets so I’m glad they are happy about that and don’t feel they have to hide good news from me. Being resentful and selfish is being a bad friend.

But if a pal is very tired, I don't do "oneupmanship". Tired is tired and I try to respect other people's tiredness levels. I don't say "that's not tired, you should hear about me". If your friend is doing that, op, they are being an immature arse.

It is different though OP, very different. She isn't 'tired' she is bone crushingly exhausted and may struggle to even speak, let alone move when you don't see her.

You sound so unpleasant the way you constantly feel the need to say she is 'harping on - so rude and derogatory. Perhaps it is all she talks about because it has completely consumed her entire day, and therefore life, and there is little else she has to discuss because chronic fatigue and ME (all chronic illness) completely alter your life, stop you doing anything, change everything, it's really devastating. If you're genuinely trying to understand, I would stop speaking about your friend like that, stop being 'that' person that compares the exhaustion of chronic illness to your tiredness/busyness. Your lives are both hard yes, that isnt a competition, you aren't 'doing better than her' for not mentioning it.

Maybe just stop scrutinizing the poor woman. I have severe fatigue due to a separate life limiting condition, I cut my hair short because I (literally) don't have the energy or mobility to deal with it when its long and couldn't do anything else in a day sometimes if I spent the energy on my hair of a morning. Maybe she feels insecure about having a hairstyle she doesn't actually want. If she is off work with CFS she likely has a lot of muscle deterioration from being inactive which will make physical activity even harder.

Ultimately, maybe just do her a favour and extricate yourself. Most people do when you're chronically ill anyway OP, because they don't genuinely try to empathise, make judgements or police the free time of the unwell as though it isn't possible to feel well on some days and not others when you have serious illnesses rather than just be happy for them that they were able to have fun or get out for a little while in a week so I doubt she'll be surprised. She can't really win.

...and don't moan to her about your stuff. Save that for a different friend.

I don’t talk it about it much though, if she’s a good friend she should be prepared to listen to you as well. Has she always done that?

I have MS. MS fatigue is a swine. I can barely lift my arm to reach something. I can't watch telly because it's too exhausting. I have to go to bed immediately and sleep it off. It's not like being very tired. I've had a baby. I know what very tired is. When I have a bout of fatigue, I long for "very tired".
I get regular b12 jabs from the GP. I try to eat healthily hides chocolate biscuit and get good blocks of regular sleep. I can't socialise without big planning around ensuring recovery space and time. Usually I just avoid socialising. Sometimes I amaze myself with what physically stressful or taxing things I can do - and I amaze other people. But it is not a regular occurrence and shouldn't be misconstrued as me faking it when I have a bad time with fatigue.

To be perfectly honest, I think I'd tell her how it is making you feel. Alternatively cut her out of your life.

I say this as someone with a condition & currently off work because of fatigue. I try not to talk about it too much as I don't want to bore the shit out of anyone. I have a friend with a severely disabled child and I'd never dream of banging on about being tired knowing what she is dealing with.

Friend, I love you but I'm worried that you are only able to talk about being tired and that cant be good for you. I'm going to limit discussing this to once a week/month but the rest of the time we can only talk about happy stuff.

What exactly counts as a full and active social life? Is she able to get out every day or just occasionally?

I don’t suppose she took the decision to give up her job lightly but she may have found it impossible to continue. Any type of job demands a certain amount of mental and physical energy and not many employers are going to keep you on if you have to take a lot of sick leave.

I have CFS and occasionally meet friends, go shopping and so on but most days I don’t leave the house, unless I need to, such as for things to do with my dc. I have to pace and plan everything, if I needed to do something that what have me out of the house for most of the day, I would then have to avoid arranging anyth8ng for the next few days to recover. Ineed to sleep most days and have to spend a lot of time lying down.

And this is with taking a lot of vitamins and supplements, without them I’d be much worse.

If she dismisses anything you say then it's probably healthier for you to put her at arm's length for a while. You don't have the emotional reserves to deal with her level of self absorption. I think it would be different if she was able and willing to empathise with your own difficult and draining situation but that's not the case and I think a little self preservation is required, on your own behalf.

I had CFS/ME in my teens. I had to leave school in year 8 and be home tutored 8 hours a week. Back then people thought it was a made up disorder. I can understand why and I would probably think the same if I hadn’t been in that position myself.

I have a friend who has another ailment and all she does is talk about it. To the point where she is starting to be excluded from things by multiple friendship groups because it’s just tedious.

I have no advice but I am following this post closely as I too would be interested in people’s advice.