Wht do u do if u start likin ur gf's bestfriend?

ight so my gf pisses me off but i love her so much its unreal, but me n her bestfriend tlk n i started to like her, i grew some n told her n turns out she likes me too i feel like an *** hidin this behind my gf, i didnt cheat or anything i jus like talkin to her, bc whenever my gf isnt there, her bestfriend is n it seems like she cares about me more then my own girlfriend. wht should i do?