I know what that's like, I went through that for about a year. The best I can tell you is to give it time. Keep your distance from them and try to forget them. I know it will be hard, but the longer you try the easier it becomes. A tactic I used: think of anything and everything you ever didn't like about the person. Write it down and throw it away, tell it to your pet (if you have 1), scream it into your pillow, draw a picture of them and shout it pretending it is them. Either way it's bad to keep it all inside. Try to find someone you can talk to about it. Believe it or not, it will help.