Welcome, psycnerd!

I'm not an authority on post-concussion syndrome but I do know that there is a real post-concussion syndrome that isn't imagined. I can't say whether you have that condition, but since you're in doubt about returning to your studies, I do have a suggestion.

If you look carefully at your university handbook/catalog, you'll probably find out that you can ease into a degree program in various ways--usually by taking one course at a time, or at least taking fewer courses than the usual student load. There might be some strings attached to this arrangement, and so you should read up on it carefully and maybe ask your adviser about how to do that.

If you could ease back into it slowly, you could get a better idea of just how much you're up to doing, and you could gradually work up to more of a work load.