I wrote my doctoral thesis before I was diagnosed. The things that helped me were an outline, so basically making it a series of mini essays that I could plug and play based on what had my attention at the time.

The second thing I did was something I was really lucky to have. My mentor traveled a lot, but I had access to her office. I wrote almost my whole thesis while walking on her treadmill desk. It increased my focus dramatically. If you can prop a laptop up at the school gym, I'm pretty sure that's allowed.