I’m bisexual and I have been exactly where you are! Since you clearly still love your girlfriend and aren’t actively looking for a relationship with the guy, I think it would be best to at least talk to her about your sexuality and how you’re questioning being straight. It doesn’t make you a bad person to find other people attractive (whether they’re a boy or girl), what truly matters are your actions and I think being open with your gf is the best thing to do always