The process of thinking about past events can be called retrospection. It will take some retrospection to remember your favorite moments from the last year or two.
You might be familiar with the word introspection, which is all about the act of thinking about yourself — the things you do, the way you are, and the things you think about. Retrospection is similar — both words are rooted in the Latin specere, "to look at." Instead of intro-, "within," retrospection uses retro-, "behind," or "in past times," so your retrospection at your high school graduation might have you thinking back on your years of school.