My concentration is not good, although I'm not bipolar...but probably ADHD, though this has never been diagnosed. I did return to school, first in my mid 40's, for teacher certification, which was intensive and demanding. In studying such subjects as science and math, I had to study in increments of time, write things down, read it aloud, and read silently, working on memorization of certain facts. For the standardized test given to all prospective teachers, I bought one of those test-study books. It helped.

In my mid 50's I returned to school for M.A. There was a lot of reading and research. With the reading, I resigned myself to reading for a set period of time, taking notes, then getting up and doing something relaxing.
Oh, I should also mention that I had to limit my classes to one or two per semester...just couldn't handle multiple demands.

I completed both of these programs, though it took me longer. The great thing about returning to school as an older student is your appreciation for your education. You are a much more focused and serious student than when young. I suggest pacing yourself, don't take too big a classload at the start till you know what is comfortable for you. I encourage you to give it a try! I think you'll be great!
Patty