To be extremely upset about my prospects?

[someoneneededyoubree]

I'm due back at work soon.

I'm back part time. It's a new position.

But I'm terrified of what the future holds for me.

I have an illness that is getting progressively worse as the years go on.

I've had every test under the sun and so far, all I'm able to have is strong pain killers such as Tremadol and Nixoproxen (excuse spelling).

I'm so young, it frightens me. What if I get fired? What if I cannot work because my illness makes me take too many sick days? I suppose those aren't even what ifs. They're eventualities. I'm not fit to work at the moment. Goodness knows what I'm going to say to my new employer.

I'll be completely honest when I say, that I used 'pregnancy related illness' as an excuse in my last role a lot, because it didn't go on record.

I want to cry at my lack of potential.

AIBU?

It's not unreasonable to be terrified like this, but fearing in advance won't help. Maybe you could practice some distraction technics they teach at therapies, like saying "I'll think about this at 9pm, but not before", thus bouncing these negative thoughts as they come.

You don't know what's in the future. You might have a great job with nice, supportive people. (Although I don't have health problems and my example might not be relevant, but I feared the restructure at work so much it made me really ill, but in the end the change actually worked out in my favour. So it tought me not to fear in advance.)
Hope you'll be ok.

That sounds hard and I understand your anxiety.
Could retraining in a career where you can work from home and be freelance be the long-term solution. A job which isn't time sensitive and can be done around the ups and down of your illness. I'm thinking freelance book-keeper or similar. What skills do you have? Perhaps people on the employment board could give advice. Going out to work isn't the only option.