Forgive me if this isn't news to anyone else but I've suffered with low level anxiety all my life and it's literally only just occurred to me and I've never heard a MH professional mentioned it either.
Because I spend a lot of time in a heightened state of emotion and my anxiety causes me to obsessively revisit things that went badly (or things I've perceived to have gone badly), I'm worried that my strongest and clearest memories are therefore almost always bad. This might explain why I feel quite content much of the time but when I think back I always think I've been unhappy most of my life. Do the bits that are just ok all sort of moosh into the background and get forgotten. What a sad thought! 