Need High School Relationship Help

Ive been well.. "fooling around" with this one guy since freshman year. I am now a junior and have been in my first long term realsionship. Lately I've found all I've been thinking about is hooking up with the other guy. My boyfriend is really into me. He says he loves me and I say it back, but we are only in high school and I just want to have fun. I like him a lot, but he likes me way more than I like him. I don't want to break his heart, but I can't help lusting after this other guy. I know if I break up with him I'll be devistated. On the other hand I know going on like this is really selfish of me. What should I do?