Rip the band-aid.
It might be 'devastating' for her,.. ( She made sure you knew that ? Hmmmm.)...but at least then, you are being honest. Nothing worse then being secretive and lying about your true feelings. The longer it goes on, the worse it gets.
Oh, and prepare for her to be pissed-off that you fucked her, knowing you were going to leave her. Lust took over for you, I am guessing she was expressing love to be with you. Bonding.
I know its hard, but you need to man up.