Not over an ex

I'm 17 and for 3 years I dated this guy that I thought I was Inlove with. We broke up 8 months ago. Our relationship was not healthy but we made so many happy memories together and it's so hard to let go. I've tried talking to other guys and even dating one, but I always leave them because i either get tired of being in a relationship or I start thinking about my ex. What are good suggestions to moving on and being happy with someone else?