[Eternal]

You aren't an awful girlfriend, in fact its common in teenage relationships. While its great that you two love each other and are pretty serious, its easy to want to experiment and try something new since you're young.
I agree with Kat in making a list of what you like about this other guy. Typically though, if you start liking someone else it means that they have something to offer that your current relationship doesn't.
That being said, it doesn't mean that you're relationship isn't going to work out. Since you've been together for 6 months you two know that you work well together (a lot of relationships that have no hope end much sooner). However, you don't know if things would work out with your crush. So its up to you whether you want to try something new, or stay in your secure relationship.
You can always try and avoid this guy you're developing a crush on and spend some more time with your boyfriend. In time, you should get over the other guy.