The lost potential is a key one I have seen. The idea that your youth is in the rear view mirror and that you didn't do all the things you thought you would do.
I was in my 40s and starting thinking more in terms of how much time I had left rather than what I wanted to do at some unspecified "later". I also had young kids and a depressed jobless wife, so I wasn't completely free to do what I wanted and felt trapped. Then my father died pretty young, from a cruel cancer, and that amplified it all.
