Mate, if you ask me, it doesn't sound like you're quite sure of your sexualilty, so maybe it wouldn't be a good idea to tell your girlfriend yet. If you really want to tell her, wait. Wait until you can be more sure about what you're saying to her. Either that, or separate. Only to ease your conscience, but it might be better for both of you...
I hope that helped, after all, that's what I'm here for
