Perhaps you're bi.
I also think your wanting someone after you've broken up with them to be kind of teenaged/Buyer's Remorse. Only in your case, you put someone back on the shelf and then think, "Hey, I kinda miss that."
I'm guessing you remain broken up afterwards, yes? I think that kind of goes as a corollary to liking girls who are experimenting.
In both respects, you don't have to give too much of yourself.
Maybe that's all this is, that you get into relationships but they aren't too serious for you. Or you unconsciously choose people (or they choose you) and it's nothing serious.
And that is awesome and cool and wonderful, seeing as you are 15 years of age. I have to tell ya, we see boys and girls within your age and they are going on and on about how they love this one or that one, and it's often after a week or even less of dating - sometimes it's just from a bunch of allegedly meaningful glances.
I think you've been aloof (despite your relationships) and you haven't found anyone yet, of any gender, who you truly, passionately love.
That's awesome. It's very smart of you - you don't fall without reason. You don't commit and blurt out three little words at the drop of a hat.
I bet you will one of these days. You might be 16. You might be 36. Shrug.
Don't settle for what you don't really want. Don't lie to people or yourself about your feelings.
Go and be awesome.