A memory you keep revisiting isn't exactly a pleasant one. Surely there are other ways you could spend your time daydreaming. What is the use of going over and over something that upsets you? You aren't going to be able to undo or change it anyway, Aquarius. It probably feels like you are trapped in that memory, and each time you vow to let it go, it comes back again - but you do keep letting it in! Try to literally replace that thought with something happier and more hopeful. That's the only way to get out of that loop.