I'm a little torn, and a little confused. Advice?

Well, I've been dating this girl (A) for a while, and it's been going great. But there's another girl (B). Now, I was good friends with Girl B long before I met my girlfriend, Girl A. The issue is that I'm starting to have feelings for Girl B. While I'd always dismissed previous notions and thoughts of attraction/infatuation towards Girl B, I have recently learned that she has feelings for me as well, making matters more complicated. But now, it's not something I can ignore. I'd NEVER want to hurt my girlfriend, and I can't imagine breaking up with her over this. But, am I truly being faithful? it's not as if I'm INTENTIONALLY having feelings toward Girl B, but...I just don't know. I was determined to figure this out on my own, and not consult my buddies (mainly because I think they'd say "Oh yeah, bro! Get both!". We're 17, and 17 year old guys tend to think like that...), But I haven't asked advice from ANY one, because I wanted to figure this out myself, so I would make the best decision. But it's not getting any better, and it's driving me bloody nuts. So if anyone can relate, I'd appreciate whatever advice you can give...honestly. If I take your advice and it works, expect an e-mail of gratitude in the future. Thanks,