Unstable emotions
I'm not diagnosed yet but I believe I have BPD (borderline personality disorder) and I've symptoms for as long as I can remember but I'm 19 now and it's bothering because I met this girl online about 3 years ago (I met her online still to this day we haven't met in person) and we've been friends for 3 years and we both realized we wanted to be more than just friends so that's what we did. After a month of being together I started to lose feelings even tho I didn't want to. I wanted to be with her and I tell her everything so I told her how I was feeling and I told this can't work out because the longer I'm with her the less I feel like I don't want her even tho ik I want to be with her. So we broke up and now I want to be with her anymore. We still talk to this day but I don't want to be in a rollercoaster relationship with her cuz that only benefits me. Any tips
Well, I'd say maybe go see a psychiatrist. Self-diagnosing is not often helpful. Once you know what is going on, you can get the care you need in order to best deal with your emotions.
It sounds like your issue with your girlfriend is a symptom of a deeper issue. So that's what you should focus on.