I've also found Anki a good remedy to the "I feel bad that I don't remember this from undergrad" problem. I'm still nowhere near as fast at actually doing maths as when I was taking or teaching it. But now when some basic, relevant mathematical fact comes up in life and I've forgotten it, it just means there's one more thing I need to put into Anki, not that my mathematical knowledge is on a steady, irreversible downtrend.