I don't know why it's so hard for me to move on and let go. Bad things happen and when I feel like I…

I don't know why it's so hard for me to move on and let go. Bad things happen and when I feel like I am finally able to move on from it and I am actually happy, my whole world comes crashing down and I just start to remember all the pain that I felt. Then I just become depressed. It's like my brain doesn't want me to be happy. :(