This all started when you were only 13! You are right that you have been discovering your sexuality. The problem isn’t that you’ve been attracted to two people. It’s normal to try out different relationships during the teen years. In fact, it’s an important part of figuring out who you eventually want to be with. (It used to be called “dating”.) The problem is that you haven’t been honest with the other girls or, especially, with yourself. With each switch, you acted as if you were making a commitment. No one is ready to do that in the early teen years. You and the others are no exception.
You haven’t been alone in the drama. It’s not all your fault. The other girls are participating in the on-again, off-again nature of the relationships. They too are discovering their sexuality. They too aren’t ready to settle into an exclusive relationship, however much they think want one. The result of your collective confusion, experimentation and intensity is the emotional roller coaster you describe. No wonder the group is breaking apart.
I think you should give yourself a break from sexual relationships for a while. The neediness you bring to relationships guarantees that they will fall apart. You need to make friends with yourself so you can enjoy time when you are alone.
Develop your interests. Take your schooling seriously. Set some worthwhile goals and work toward them. Then work on developing some solid friendships. Spend time having just plain (non-sexual) fun with people. There is no reason to get so intensely involved at your age.
If you can’t do this on your own, I do suggest you see a therapist. It may take more than personal resolve to get out of the pattern of trying to fill whatever hole you have inside with intense and probably inappropriate relationships.
I wish you well.