ouch man, theres no easy way out of this one.... except maybe a threesome, but somehow i dont see that happeneing.
Uh anyway, stick with your current girl, at least for now. Unless you like trying to keep a long distance relationship going too. Plus your best friend girl was probably a little tipsy that night too, so just kinda pretend that nothing ever happened, or talk to her about it and just say that it was a one time thing, and forget aobut it. no need to totally stress yourself out over this. we all screw up eventualy.