Ok, I'll start by saying that I have the most amazing boyfriend in the entire world. We have been dating for two years now, and he is seriously my best friend. We talk about everything from past relationships to our future house we wanna buy someday. Were even talking about getting married my sophomore year of college... However, he has a best friend... who I am extremely attracted to. They are currently sharing an appartment together, and I feel so guilty because half the time I only want to come over if I know he is there. Whenever he is around we are always talking, laughing, and having a good time. I don't know if its just because I'm feeling bored with my current relationship, or if I'm just regretting never really having an opportunity to just date around, or what. But I really don't know what to do, because the more I'm around his friend, the more thoughts I have about being single, even though I know if I become single, I would probably lose what I have forever. Any advise?

Yes, you should really think about what you have and again think is he really worth losing a best friend? And maybe you shouldn't hang around the place?
try meeting him some where close. but not around the freind, or talk about it.