How do you forget an Ex?!

I met this girl 1.5 years ago, during summer, and I fell for and ended up asking her out and she said yes. So after our date, she told my best-friend (who she knew for a few hours) that she would've rather kissed him instead. I didn't kiss her because I nearly got stabbed -(seriously :p) - and I was not in the mood. Long story short - she kept flirting with him, told him horrible things about me (according to what he said), and he ended up asking her out (she said no). My 'best friend' wasn't exactly nice about it either... he kept saying how he wanted to ask her out etc.. I remained friends with her for a few months after and we just stopped talking after I confronted her about flirting with my friend. So now, about a year later I still miss her and I don't know why!! She said happy birthday to me out of the blue, and I started to think about her again! We didn't even kiss! (We did have some intimate moments though) and she was the first girl I asked out/ went on a date with. How do I get her out of my head?! It's killing me thinking about her! And I really don't want another relationship until I'm in uni (few months from now). And thanks for any feedback. Sorry this is so long :p

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