I've felt a similar sense of loss about losing knowledge picked up while in academia. I think that to re-learn properly, however, you need to find a way to apply the knowledge "in anger". There's a reason why we did endless problem-sets in school.

After a few fits and starts at re-learning I've found the only things that stick are things that I end-up using (albeit sometimes in a forced way). Nothing wrong with a nostalgic perusal of classic well-written texts, but these kinds of things were never intended for just reading. You gotta apply it to really know it.