What should I do, Should I just forget it?

I dated this girl when I was 16 and then I was a stupid idiot, i ended up dumping her I'm 19 now and I cant stop thinking about her, now that Im older i realize what a great relationship we had, she was the best she understood me she was so nice and i felt so comfortable with her about anything, we havent talked in about a year and a half but now she has a boyfriend shes been seeing for a year or something, and now she really loves him, i was her first love and she was mine :'( What would you do?