I've theorised that because I am very sensitive to what other people appear to think of me, I've tended to remember in particular those occasions that most relate to that concern, both positive and negative. For instance, the nursery memory is my earliest memory of making people laugh, which is a positive (ooh, people like me!).

Your description of constantly replaying the unhappy memories though - that sounds rough. Reminds me of compulsive rumination which can be associated with OCD.