Still have feelings and can't get my ex-girlfriend out of my mind, what to do?

My ex girlfriend broke up with me about 3 month ago after being together for about 8 months. After the breakup she told me she still loves me and wanted us to be friends but wanted to be able to hangout with other people. About a week after the breakup I saw her together with her old boyfriend how lives in another country. We went out for dinner a couple of times, at this point the old boyfriend had left, where she touched me a lot, kissed, and we ended up having sex. The last time we went out for dinner she was going to sleep at my place, we would have had sex again but she got her period. We started to talk about our issues as a result she went home, said she had to protect herself? About 3 weeks ago I got a email from my ex (after about a month of NC, we have never communicated though email before) with subj "Help". She asked me not to tell her old boyfriend that we had sex after he had gone back home if he contacted me and that her life was falling apart. I replied that I would/will never hurt her. She then replied "Thanks, that means a lot..". Later got a text (this time on WhatsApp?) where she said her old boyfriend was hurt and wanted me to know that he knows we had sex.

Send a couple of texts to her about 2 week ago, got 8 very angry text back telling me to stop contacting her. Says she wants to spend the rest of her life with her ex and she is trying to make things right with her ex. Even got a email which I wasn't suppose to answer, explained that we where never together and only had oralsex. Later got a message from her ex boyfriend how she is visiting asking about the relationship status between me and me ex girlfriend. Sent HER a reply explaining why I sent the messages, got a positive reply "I know your only mean well". Haven't been in contact for about 3 weeks and want her to know that I can't be just friends, don't know how to let her know or if it even matters. Want her back but I know I have to let her go, don't know how.
Still have feelings and can't get my ex-girlfriend out of my mind, what to do?
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