Hmmm, but I think it would be more hurtful if you stay in the relationship and fake your feelings about the relationship. Soon or later, things could get very ugly (if you stay) and she'll dislike you for life. If you care about her, be honest with her. You owe her at least that. And like likemclever said, if you don't find someone right away or even for a long time, don't go back out with your girlfriend to fill a void or to satisfy your sexual needs.

It might be best after you talk/break-up with her, stay away from her for a while so you can both adjust to being single.

Good luck to ya!