What do I do?

I was 16 when I met her we went to high school together and she was the most beautiful girl I'd ever seen. We were together for 4 years and at first it was the best thing that either of us had ever experienced, but then things started getting out of control and it became this crazy roller coaster of love and hate and I did some things that I'm not particularly proud of and she never got over it. We actually had a falling out for about a month or so and at that time she started dating this one guy and I spent my time with a girl I worked with. After awhile, we started talking again and soon it was like we were in love again. Shortly afterwards, we would have the best summer vacation of our lives. But as does everything, it naturally didn't last. That year she went off to college and we tried to make it work, but that didn't last too long and she broke it off. She ended up dating some other dude and I was left holding my own. Its been almost a year since then and I still think about her everyday. As far as I know she has been dating other guys and what-not. For the past 10 months I have been single and lonely and unable to get over this. If there is anyone out there that can help me and give me advice, please I need it more than anything right now.