What do I do I still love my ex girlfreind?

6 months ago I asked out a girl I really fancied and we got quite serious. We did everything but sex even though she wanted to but I didnt feel she was emotionally ready. Her mum never really liked me and after about 2 months she started making this very obvious and this really upset my ex. She wasnt trying at school because she wanted to talk to and see me and the only thing she cared about was me. Then two of her friends told me that she liked another guy. Usually I wouldnt have cared because she had liked lots of other guys. But this time I thought it would make her happier to be with him. I dumped her even though I didnt really want to and managed to suppress the depression and started dating another girl. But I wasnt happy even though she was gorgeous because I wanted her to be like my ex. A month after me and my ex broke up I found out that she still loved me and was up everynight crying and so I wanted to ask her out but I was still with the other girl so didnt. But then one day at school my girlfriend was out on a trip and me and my ex were getting on really well and kissed infront of everyone on the bus which ment by the next day everyone knew and I was dumped but my ex wasnt. My problem is I still love her even though that was 3 months aago and I can never sleep and can only think about her. She keeps me holding on by flirting with me and things like that but I need to let go. What could I do to let her go?