You broke up with her and she kissed someone else. She doesn't have to and won't regret this the rest of her life; it's a trivial thing she did and she didn't do it to you. You dumped her and she reacted. It's part of dumping someone, that the person you dump may lash out or run out and do something sexually with someone else? If you can't stand the thought of kissing her anymore, and you already dumped her for (lol) feeling you were no longer meant to be, perhaps leave each other as ex partners and move on?