Fleeting thoughts, constantly. Spending a lot of time bringing myself back from mini-tangents inside the brain; in mid-sentence, distracted by thought/sight/sou

Fleeting thoughts, constantly. Spending a lot of time bringing myself back from mini-tangents inside the brain; in mid-sentence, distracted by thought/sight/sound/smell and hence lost train of thought (dozens of time a day). I seem to do reading fine, but retain very little from the reading, in comparison with classmates. I've mostly been taking math-y classes (cs, math, phys). Physics was very difficult. Writing essays is even more difficult - this is the source of most of my academic troubles.