Hey Oberon,
I'm also finishing my undergrad BA set to graduate in two weeks. It sounds like, if you're working on your thesis, you're at the finish line, so it may be important to start gearing towards a career path that is accomodating and flexible with your fibro, as I am trying to figure out now. Since you're asking about advice for school, I will offer that here rather.
Fibro hit when I was in my 3rd semester of college so I took two semesters off before coming back and switching majors out of engineering, a field that was in my opinion too demanding with stress and thus wasn't sustainable in function with fibro. I finished the minor in computer engineering but I switched to business (Finance). Once coming back to school I was full time, taking 4 classes per semester, for 3/4 semesters. About the same time covid hit I went down to part time and have done 3/2 (even 1) class per semester. This also worked out considering the classes I took part time where higher level classes that were more difficult. Throughout my college career I think I took 4 / 5ish incompletes, whereby I didn't manage the class in one semester, and asked the teaching professor If I could start it over next semester or continue where I left of. In one case I finished the class just needed more time finishing the final project. My university's policy is quite strict on incompletes and they are stated as only for medical purposes i.e. serious injuries, so getting incompletes for fibro, a pre-existing condition that didn't occur in the middle of the semesters when I took incompletes, was a little tricky. Esp. considering how fibro is still not seen as a very palpable condition unfortunately - AND I don't have a formal fibro diagnosis. But the professors have final say, and I found just explaining the situation to them and talking it out convincing them was enough. In one case, I met with the professor a few weeks before the class started and explained that I had this condition to start on the right footing, and it was indeed helpful because she was more flexible with me with deadlines and gave me the incomplete to hand in the final after the class was over. If you need it, I do recommend looking into incompletes.
That's the overview of how I handled college with fibro overall, but inbetween I also picked up lots of coping mechanisms and methods. Covid-19 made virtual class very manageable, and although I still opted to go in person to classes as much as I could, I would take class virtually whenever I was in a pain flare-up (The classes were hybrid in person and virtually telecast). As others have mentioned, don't compare yourself to others. It's really easy to but even taking 1 class a semester is an achievement with fibro.
For the brain fog, I would recommend a number of things. First, sleep well and get in a steady good routine. Second, eat well and adopt healthy staples. For a long time green tea was a strong staple of mine, providing just enough caffeine to get me going but not too much to cause more tension and pain. I've recently switched to hot cacao with a touch of high-quality coffee grind that doesn't seem to be causing more tension. At one time I was taking wellbutrin, which can help with fibro fog by pushing the dopamine system to deliver. I don't anymore but I recently started vaping with nicotine, which could have a similar effect. What was really notable for me was stimulants; years ago I tried some that interfered with sleep and didnt help much but I found one that has worked well and I've been taking on a low dose for two years now (concerta). Note that I have ADHD diagnosed since way before fibro so. Try meditating as well, although I know it can be hard to make it a common practice.
I've been where you are where you think that delaying one more semester would have been better, to deal with and try to better manage pain symptoms. But I've also found that not having a purpose, responsabilities and a schedule to abide by in my daily life can be quite detrimental to mental health at least in my case. Too much of course can also be bad, but I think the best way to manage is a steady pace if reduced, rather than taking entire semesters off college because of normal fibro developments (without new or complicating developments).
Hope this helps.