Hello Jeremy
I am very sorry that this has happened.
Do make sure that she hasn't passed on a diseases to you, if she has been sleeping around.
As I said before, for some people, sex is just something to do; for others, it is personal & meaningful & special ~ and I get the impression that you belong to the latter group.
You know that I think 15 is too young for a sexual relationship, but I also know that this sort of thing happens. Put it down to experience. You are learning about people and relationships.
I know that you yearn for affection, but the right sort of affection will come your way, when the time is right.
You say that this sort of thing keeps happening, but you are still very young. I didn't even have a boyfriend until I was 18. Boys asked me out, but I always said no, because I wanted to be sure that I had found the right one, for whom our relationship would be real & meaningful; someone I could trust, as well as love. When I found him, I stayed with him.
Not everyone does this. Many feel the need to experiment or experience ~ and that's fine, but don't use or be used; be sure before you commit.
Time is on your side. When you are a little older, you will have more independence and you will be mature enough & experienced enough to make better decisions. Don't expect to get everything right at such a young age. You may be growing up now, but, as yet, you only have the knowledge and experiences of a child to look back on.
Take care and stay positive. Many young men are going through similar emotions to you ~ and girls too. It happens a lot. You can tell that from some of the posts on this part of the forum.
You can always come on here if you need to 'vent'.