Speaking as someone who's going on 38 next year, I actually agree with Lilith. Memories can be your worst enemy as you get older because they can grab you and make you relive parts you don't like over and over. This isn't the same as learning lessons from bad experiences; this is your brain insisting there's still something you haven't learned and force-feeding you it. If you ever wondered why some crazy old people can be so crazy, that's why. Being tortured by your own heart warps you just like you found your hole. The hole that was made for you.
I really do think the key to happiness is to forget the bad things, but hold onto what you learned from them. That's the only purpose bad memories serve, in fact -- lessons and reminders of people you owe. (whether it be because you hurt them or they helped you out of a bad spot)
If you've learned what you needed to learn and you've settled your accounts, if possible send painful memories to the bin and never look back. If only it were so easy.