Most Helpful Girl
It sounds like the best thing to do would be to break up. But only you know how you feel. But to answer your question on how to break up with him. Just be honest about it. He does sound a little bit too attached. Just say something like, "I've been thinking about our relationship and I think it's moving a little too fast. So, I think the best thing to do would be to break up." Just explain things a little bit so he doesn't feel like he's just hanging.
You said you were worried about him thinking you were a whore/being a jerk. You don't even have to tell him about the other guy, it really doesn't matter since you said you weren't comfortable with the relationship anyway. About the being a jerk part, put yourself in his shoes for a moment and imagine the emotions he would go through. Think of comforting things to say to him and give him a minute to take everything in instead of leaving immediately after telling him. Hope this helped and good luck with everything. :)