PTSD? Were you just overwhelmed by all of the derivatives you had to memorize and the rules for integration? It can be a little overwhelming at first. I always liked u-substitution. It felt like a puzzle. You’re absolutely right. I think when you’re in your twenties, you’re filled with so much how for how you’re going to apply all of this newly acquired knowledge to your future job field, and then after thirty, the realization sets in that you might not keep advancing in your area of study; instead just be applying years of prior knowledge to complete tasks and train others. I mean, consider complex analysis, it’s a trip, and I could watch some cool videos on YouTube to relearn it all, but at age 35, I find myself wondering what else I’ll do with it, or if I’m neglecting some household chore, or errand I’m supposed to be doing.