You may love this girl, but you're not longer IN love with her.
You need to end this relationship. Its unfair on you and its unfair on her. It sounds like you'd end up cheating on this girl if/when you meet someone new and who you do fancy. I think you need to break it off with her now, she deserves your honesty.
You never know, in time, you may be able to still enjoy her company and be friends with her, but don't expect this straight away and it may too difficult for her or even both of you to do right now.
Hope this helps. x