When a guy stops showing interest, it’s either because he doesn’t like you as much as he first did or because he’s met someone else he likes better. Either way, it’s tough for a 20 year old, like this guy you’ve been dating, or a guy at any age, to tell you why he’s rejected you. He doesn’t want to hurt your feelings, and even though he has, he doesn’t want to see your hurt. He just wants to move, and he’d prefer you be okay with it all. Of course, that doesn’t mean you have to!
I know it’s disappointing to be rejected, but dating is a way to get to know people, and when you do get to know each other, you find out about them — and yourself, and yourself with them. You may not like everything you learn and you may see incompatibilities that make you want to move on. It could be that one of you wants more than the other, one of you isn’t that fun or lives too far away, or simply because one of you isn’t as interesting as someone new you’ve met. Because he’s not being frank with you about why he’s moving on, you’ll probably never know the exact reason. But the reality is, he’s moving on, and you’ve only been dating him since November — so there was always a chance something like this would happen. I usually recommend that you use the first six months of dating to decide if you want to be monogamous — and if you use that rule, you’ll understand that this kind of thing may happen, so you’re not so hurt.
I hope that helps.