In April my relationship with this girl ended. That broke me, and I found out a lot of stuff about myself. I had to consult a psychologist to get myself in line again. This was not just because of the breakup, but underlying problems that were exposed because of that breakup as well. Around october I felt great again. I was over the breakup, and I didn't often see my ex-girlfriend again. I even had 2 chances on a F-close but I didn't really like those girls.

In november me and my friends were going to a festival. Happens that my ex also came along. I didn't really mind. But this is what happened. Me, my friends, my ex, and her friends all used MDMA. It makes you feel euphoric. In that state me and my ex kind of started touching eachother again, on the hands, arms and back. In the end we kissed. Since then we're kind of dating again, but after 1,5 month I found out I just can't fall in love with her again. It doesn't feel right.

However, I can't push her away. I'm so terrified of the fact that she's going to be seeing other guys. Im sure, VERY SURE, that she is NOT the girl for me, but somehow I can't let her go. I'm sure this is a form of oneitis, becuase the reason I can't let her go is that I'm afraid that I'm not going to find someone like her again. I'm not sure what to do now. I'm also afraid that I'm going to fall back into the same ''depression'' i was in 6 months ago. Any advice would be appreciated.

You have deeper issues that could never be covered or dealt with properly in a forum discussion. In general you are suffering from a form of codependency, which could stem from any number of things too long to list. The best thing you could do is to continue to get professional counseling. You could also try working on improving your own situation by increasing your abundance of opportunity with women, which will help work against your scarcity beliefs. But the reason a counselor is beneficial is because they help you connect the dots with your past experiences that led you to have such maladaptive beliefs and identity issues. So in summary my advice would be:
1. try to gain as much self-awareness as possible and understand where your baggage came from. You can technically do this on your own, but a good therapist will speed it up.
2. improve your current situation as much as you can. I.e. meet more women, date more, and find other means to increase your happiness outside of women.
3. avoid continuing your current patterns of thinking that trigger your grim outlook and negative mental-masturbation. In other words, actively try to see the nice things in life and stop dwelling on hypotheticals like who your ex is dating. It's no longer your concern.