Did you ever tell anyone, the police about this? If you did, was it resolved? If not, that might be the first step. Only thing is, if there isn't enough evidence to please the police into an investigation, they're reluctant to do it... :/
Generally speaking, letting go of past events is rather difficult especially for most of us on this forum. I could also suggest trying to introduce new things into your life, to hopefully "push out" those painful old memories by putting happy new ones in its place
