What do you do when you have a boyfriend, but are really in love with someone else?

It is only fair to your "bf" that you are upfront and honest and break it off with him. It isn't right to keep holding on to him and stringing him along if you love someone else. Be honest and let him go find someone who truly loves him.

Break up with your boyfriend.

If you are no longer in love with your partner then you do both him and you a disservice by pretending something that isn't there.
But also be aware the difference between a crush, being 'in lust' and being in love - this 'someone else' might just be a temporary distraction. If you genuinely love this person - even if for whatever reason it wont ever happen between you - why would you WANT to be with anyone else?
Whatever the case - sounds like someone is going to end up hurt, (please note: cheating is not the answer to ANYTHING) best thing you can do is be 100% honest.

oh come on, how can't you know? break up with the boyfriend and then date the other guy.

These things usually end badly.

If they guy you love is free, then get with him. If he isn't, then love the one you're with.

Go for love... I know easier said than done, I love my bf with all my heart, he is stubborn, self centered and he makes me crazy but I could not imagine just being in love with him from a far! I did that once and I felt like I was dead inside

you have to true to yourself. i mean you cant truely happy if your not. Your bf does have a right to know about it to so i would talk to him

You dump your boyfriend.

I would say it it time to move on, the longer you stay with him the more his emotions aside. Plus if your not happy then go with what does make you happy.