I know it's really old fashioned but I'd suggest just reading a book. Tonal Harmony, by Kostka and Payne is a good, standard textbook, and inexpensive, used copies are available from Amazon (and elsewhere). There are on-line courses, too, but for me just going to the book is faster.
Your old teacher was quite right, knowing theory gives you all sorts of shorthand for memorizing bits where your muscle memory might fail; you'll remember chords and progressions in clumps rather than as individual notes, and it will solidify the weakness in pure muscle memory.
The plain fact is that music theory is incredibly boring and depressing to learn. If you are paying someone to correct your exercises, you at least have the motivation of wanting to get some return on your investment.But if you try to learn music theory by yourself, you will probably do what almost all ordinary, healthy people do: You will find it so appallingly depressing that slowly and gradually, you will convince yourself that learning it isn't really necessary.
A music theory textbook sounds like a good idea, but I found it much easier to stick to the course, and use it as a foundation further ahead. If you are already familiar with the material in the course (which I believe is roughly the equivalent of first semester music theory), feel free to disregard this.
Just wanted to add that this has not been my experience at all. Learning music theory made everything "click" for me, and I have not looked back since. It made my playing so much better when I stopped fretting about chords and started thinking in terms of chord function, contrary motion, etc.
Thank you all so much for taking the time to respond. I will probably try out some books and online courses and see what works best for me. j_tour, would you mind elaborating on that part in your first post about combining what you know to a different part of the brain? I'm afraid that bit was lost on me.