Dear Hurt and Confused,
Unfortunately I understand all too well what you are experiencing because I had a similar experience when I was in high school. Although I did not have sex with the guy, things were very intimate and very UN-committed.
I too started out dating an older and what seemed to be the perfect guy. My parents loved him, I liked him a lot, he got along with my friends, he treated me well...at least I thought so in the beginning, etc.
Unfortunately, I was unwilling to sleep with him and his hormones were raging so he broke up with me. Within a week I was getting phone calls and emails about how much he missed me etc. And within a few more weeks I had agreed to resume our relationship but on different terms, no commitment and strictly casual relationship. This was a BAD SITUATION!
Maybe you can relate to my situation as I feel that I can relate to yours. There are a few problems I have with this situation. 15 is very young to be dealing with ALL of this and probably the reason why is because this guy is older.
Cut Ties With Him
If this guy is more than a few years older, my recommendation is not only to cut all ties with him permanently but also to avoid dating guys who are in a different place in their lives.
Having said that, usually, guys who date girls who are much younger them, based on my experience, tend to be immature- (this in no way means that you are immature, in fact it probably means that you are more mature).
He Stopped Getting What He Wanted
The reason why he called you a slut is because he stopped getting what he wanted from you because you got wise to the situation and realized that it was a bad scene.
Whether you had sex with him after one day or three years doesn't make you a slut. HOWEVER, guys do tend to view girls who are easier to get into bed as sluts.
You're Not A Slut
It all depends on how you define "slut" my understanding of the word is someone who is sleeping around with multiple guys regularly and has no self respect. It sounds to me, based on your decisions and actions, that you do not fit this definition.
Unfortunately when relationships end, people tend to be left with questions and hurt feelings. Sometimes it is both people but usually one person feels it more than the other.
In this case, you are left with a lot of questions and hurt feelings. Honestly, it sounds to me like you are better off without this guy.
Good luck and hang in there. Try to stick to guys within a couple of years of your age or the same age and be wary of guys who don't want a commitment...they are most likely going to hurt you.