Hey there. Well of course you don't want to think of marriage yet, or settling down at all if you are only 18. I did not start dating until I was 18, and went through boyfriends like tissues.
If you ex has a girlfriend, what he is doing calling you up and telling you he misses you? That isn't very fair to his current girl, which I would remind him if you talk to him again. I think that since he is leaving soon for college, he is probably getting a little scared at the thought of so much change, and wants to go back to somone who is familiar.
But you're about to embark on your own journey too, where are you sure to meet a lot of different people, not to mention a few straight up hotties. So I wouldn't dwell too much on the ex or anything from your past; just keep moving forward.
You don't need to feel guilty for not wanting to have a boyfriend right now. Just don't drag out the break-up. He'll move on and find someone else. It is never fun to have to disappoint someone, but life goes on. Staying with someone just to avoid hurting them is the worst possible thing to do. I do not think you are incapable of loving - you are just young and don't know what you want to be yet, or what you want to get out of life. I never loved any of the guys from the past, although some did have smokin' bodies.
Just concentrate on being young, having fun, and finding yourself. That's all you need to do