To feel devastated by this news?

It's no surprise you're feeling devastated, OP. It's one thing when you've been dealing with it your whole life as I have, having never known anything different, but I imagine coming to terms with it after a previously clean bill of health must be incredibly hard. I'll leave the discussions on differential diagnosis to the many brilliant PPs who have already covered it, and instead answer as if it was a certainty. I think this applies to many chronic illnesses, anyway, so I hope it might be helpful to hear.

It is hard. As you've said, you can have a few good weeks before it hits again, and those cycles often feel relentless. But it's not set in stone. I have friends with fibromyalgia, and another with ME/CFS. They don't just have good days and bad days, but good months and bad months, and good years and bad years. During a bad time it can be as you describe, only a few good weeks at a time if that, but during better times it becomes a lot more bearable. Not easy, but bearable. Catching any other illness can make fibromyalgia flare up, so I imagine the chronic UTI can't be helping matters, but it's unlikely to stay exactly as it now for the rest of your life. As well as physical illnesses, it's important to keep on top of your mental and emotional health too. I'm not saying it's "all in your head", I know fibromyalgia is an illness with very real symptoms, but stress or depression can trigger flare-ups with many chronic health issues. I imagine it feels impossible in the wake of a very difficult time, but accessing support will help you feel better overall. Some kind of talking therapy could also help you process your feelings about the diagnosis, as well as hopefully improving your mental wellbeing to prevent stress-induced flares.

Sometimes illnesses like fibromyalgia can be a case of swings and roundabouts. If you can find coping strategies and treatments to prevent one symptom you might end up developing another, but it's a case of figuring out which elements you can best handle and which treatments work best for you. No, fibromyalgia is technically considered incurable, but that doesn't always mean untreatable.

So no, I don't think you're being unreasonable at all, you're responding to a really difficult and traumatic time. Give yourself permission to be upset, and angry, and devastated. Give yourself the grace to grieve.

Whentherainwashesyouclean I had the chronic uti for a similar time before starting treatment and I really empathise, I'd never felt so low. Are the antibiotics helping?

My sister got diagnosed with this after splitting from her husband. As soon as she got past the grief she suddenly was recovered - no issues again. It’s a label given when they don’t know what’s wrong, and it’s usually a mental illness cause

This is exactly what's happened to me- covid last July, felt awful since with the symptoms you've described.
Sorry I can't be much help- I've been to the GP, had blood tests because I keep succumbing to viruses which liiiinger- and bloods all fine. I haven't pursued it further as I know they won't be able to help.
I'm going to be completely honest- isn't fibromyalgia just a diagnosis of exclusion? I've always been very sceptical of it, yet here I am!!
I don't mean you're not ill- believe me, I know you are, but my take is that it's more a post viral "thing" and it's put us at a very low threshold for anything and everything, and I don't think it helps that you have a diagnosis that's not even proveable test-wise but has sent you into a panic! That's only going to make you feel worse. And there's a very definite emotional side to this- and I don't mean just being pissed off with being ill.
Obviously I'm just speculating and we can never rule out other causes- but gosh! your symptoms mirror mine.
I'm going to have 'flu and covid vaccines this winter- didn't have either last year and was v unwell over xmas and some months after.
I honestly think it's covid that gave me a battering last year and now my immune system is playing catch-up- and getting beaten back down again by various other viruses which I'd normally have better immunity against- hopefully the vaccines will enable me to catch a fkn break!
So I don't even mean long covid per se- I just think it's a battered immune system that can eventually recover.
I wish you well.

Try some anti depressants.

Well it looks like antibiotics forever for you. Sorry to hear that.

I was also diagnosed with fybromyalgia and like you I was devastated. I saw two rheumatologists and numerous professionals as I found it very hard to accept. I had a young family and was working albeit with a lot of time off sick. I was later diagnosed with CFS also. I had a very difficult few years before I was able to start to get a hold on the conditions. I have had a lot of support, I attended a course at my local pain clinic it was one to one with a physio/OT which helped in my understanding, attended the local pain group meet ups when I could and I am currently attending an online course with the local ME/CFS clinic which is reinforcing pacing and calming the central nervous system through breathing and meditation which is also what the OT at the pain clinic recommended. I would say like other posters to make sure you have not had another illness missed/misdiagnosed, to see a rheumatologist for diagnosis of fybromyalgia (if you haven’t already) and to get your vitamin levels checked and do some research on what they should be. People with fybromyalgia are prone to low vit D, my gp told me mine was slightly low but at the time I was seeing an osteopath who was trained in nutrition and she explained that it was actually very low as guidelines are not upto date. Taking vitamin d and vit k made a big difference at this point as low vit d was causing me additional pain more time off work it felt like the pain was in my bones it was terrible. I still take the vitamins recommended by the perrins technique which is for CFS but I feel they help me overall, I cannot say if they will help you as they are recommended for CFS but they help with making my overall wellbeing feeling slightly better along with magnesium at night and an iron tonic in the morning for energy. I also take low dose naltrexone which has made a huge difference. I still get flare ups but they don’t last as long. I still have to pace and rest an awful lot but I’m coping a lot better. I’m still in work but working from home mainly (ask your workplace for an access to work assessment ) and no longer having prolonged periods of time off work due to flare ups. If I had to physically go into work every day I wouldn’t manage. If your role at work means you cannot work from home see if your workplace can offer you an alternative role. If work are difficult seek advice from your union if you are in one, you will be covered regarding disability discrimination legislation if work are difficult. I still can get set back quite easily, any virus/additional illness will really knock me, and recovery is harder than it was so it’s possible that your recurrent UTIs are contributing to you feeling bad, I would push for a referral to a specialist for that. A late night will set me back or any increase in activity in my usual routine, I gave up alcohol as it would take me days to get over a small amount but that could be the CFS as it’s linked to alcohol intolerance. I have also changed my lifestyle a lot I really only try to do what’s necessary now. I got a cleaner as I have to chose what I use my energy on. I go out if I have to and it’s family related, don’t see friends much but I keep in touch by text and they understand. I do regular facials and look after my skin which obviously doesn’t help with fybromyalgia but it boosts my mood and makes me feel better! It can be lonely but there is support out there, try to get a referral to your local pain clinic and read up on techniques of management. Look at vitamins and low dose naltrexone. I am the same with exercise, short gentle walks is all I can manage. I did private Pilates for a while as I couldn’t keep up with a group any longer the teacher was great we did very very gentle movements but it is an expense I can’t manage at the moment. I also go through bouts of having acupuncture which I find helpful but make sure you get a good practitioner. This helps especially with headaches and chronic pain (another diagnosis). I am seeing the physio at the CFS clinic soon who runs a gentle exercise class online. Duloxetine was helpful for a long time but the side effects became too much and it lost efficacy. There are other pain meds that may help talk to your gp. If you are on anti biotics long term I would look at a probiotic also. Sleep helps but unfortunately mine isn’t good with these illnesses but I notice a difference when I’ve slept better, try to relax at night no tv etc and hot bath this will set you up for a better nights sleep. Epsom salts are meant to be good, can’t say I notice a huge difference but I use them anyway. Elemis muscle soak as a treat helps with pain (it’s expensive so only bought as a treat) Hot baths for the pain in general (do gentle stretches in the hot water ) and a heated blanket to help with pain. Good diet like you already have. Out of everything I would say the low dose naltrexone made the biggest difference but it doesn’t work on its own it’s a combination and pacing is very important. Good luck you will get through this . Sorry for the long post but I’ve been where you are and I remember the feeling xx