I graduated ~3 years ago with a CS degree and while I probably could have done most of these questions from sections 1-4 at that time, now all this looks very d

I graduated ~3 years ago with a CS degree and while I probably could have done most of these questions from sections 1-4 at that time, now all this looks very daunting. Just figuring out how to setup the differential equation for Q1 took a long time and I could, almost physically, feel my thought processes struggling. It's just such a terrible feeling, knowing at one point in the past you would have been more speedy with something and noticing what time has done to your skills. Do any of you have any strategies for not loosing knowledge? Would re-reading textbooks every couple months be a solution? How do I decide if it's even worth - I don't necessarily _need_ this type of knowledge for what I'm doing now but loosing skills you know you once had feels awful.