It is possible to love someone but not want to be around them. There's more than one kind of love, and relationships can change faster than feelings. Putting it as simply as some people have above is not helpful.
There are few details, but it sounds like she's clingy and immature. If so, she's not ready for a serious relationship, and she won't get there by you staying with her. You would be enabling her to avoid facing her problems. This should make it easier for you to face what (sounds like it) must be done.
If you back out, you have to do it completely. A clingy person will look for any indication that you want her back, and she'll manipulate you into making it happen. Even if not, you won't be doing her any favors. Don't speak firmly and then back down.
Don't be an ass about it. Remember that everyone can be foolish- maybe even you someday. Just be man enough to do what's best for her, you, and everyone around you both. If you come from that perspective and give it a little thought, the words will come to you. It's ok to use a script if you have to- it beats saying the wrong thing.