I just had to answer on this one, because your story seems so similar to mine in many ways. I'm 28, and I've dated someone almost always since I was 16. At first there was a 4 year relationship in highschool (yeah, the same number of years...). That ended for several reasons, one of them being that he didn't ever want to have children, but realistically, also that I had crushes on other people. I thought I can't really love him enough, if I'm having those crushes. Then approximately the same thing happened with another guy. It ended after 2 years, when I cheated on him and then left him, since I thought again I can't love two at the same time. After a short relationship with the guy I cheated him with, I met my husband. I thought that would have been the end to the problem, the fact that I couldn't just love one person in the long run. But no. I had a huge crush on his best friend, and started to think there's really something wrong about me. I started to look for people who have this same problem, and found polyamory. And realized many don't consider this ability as a problem. My husband was really great, when I told him I felt I'm polyamorous. He, in fact, realized he's at least somewhat polyamorous himself as well. I then finally told about my feelings to his friend, who was extremely surprised, but very much interested to try it out. It didn't really end well, as he finally realized he's not ok with polyamory. For the short while of few months I was with both my husband and him, I felt so happy. I felt I couldn't ask for anything more from life, and that polyamory was the magical answer to the problem. Now I'm not sure anymore whether I consider myself being polyamorous as a problem or as a great ability. So many people are not polyamorous, so there are great chances of falling in love with monos, who will not understand or be able to live polyamorously.
Also, one thing I'm wondering is that, what happens if a person like this (polyamorous but so far for 2 only at a time) stays in a long relationship with 2 people at the same time. Will there eventually be a third one too?
Also I've been wondering whether it's actually possible to be not polyamorous, but a serial monogamist by nature. I so don't want to be one, but sometimes I'm afraid I am one.
Sorry for such a long story in your thread, but you were looking for some sort of similar people, so I felt I had to raise my hand up