Um, YIKES... That would be, like, every relationship I had between 1986 and 2001.
What happened was I had to break up with a girl I truly loved, but the relationship had turned toxic and was destroying us both. I rebounded and fell HARD for another girl after that, and it took a good year and two attempts at a relationship with her to finally put it to rest. In between times, there were some minor flings, but nothing really meaningful. That's when I really started to grow up and begin to shake off my obsessive tendencies. I had another fling in grad school, and a friendship that quickly became something more. Again, I was forced to break up with her. Nothing bad. The relationship had simply run its course. We both had a good cry, and just like that it was over.
Been with my best friend off/on for most of 17 years. Coping/letting go is still difficult at times. I handled it mostly through maturity, patience, and experience. I learned that getting attached and overly emotional aren't helpful, so I focus on keeping my emotions out of the nuts/bolts of my relationship. I don't mean become an unemotional robot or a Vulcan. I just mean I don't base my decisions on emotions or whims. That has largely helped keep relationships in proper perspective. I do feel an attachment to my best friend. I've just learned how to keep it healthy, as opposed to toxic and codependent.