Yeugh, she essentially cheated on her boyfriend there with you, um, trust me I don't think you want to date that. Just think about it, if she did that with you, who knows what stuff she will do behind your back if you two ended up dating. I'm going to be flat out blunt now, apologies if I anger you:
Best advice I can give is that its time you consider some things there bro, for starters she just used you and toyed with your emotions there by kissing you and leaving you hanging like that which is a major ******* move, do you really want to be friends with a person that does that? Personally I think you should tell her straight what she did is wrong and you're done with her until she apologizes for what she did, or just flat out leave her be and find someone who actually values your feelings.
Secondly just know that if you confess to her its most probably going to end with rejection and its going to be much more awkward than it is now, possibly hurting the current friendship a bit, since she already rejected you by shooting you down after the kiss. So fact is, you're not getting this girl and I'm sorry for that man, but you'll find someone better I promise.
Ultimately, what I think would be the best plan of action is doing what is best for you and not her. If you feel you can move on better by telling her and getting it off your chest, then do it and break contact for a bit to let the wounds heal, if you feel you can move on better without telling her a word then do that instead.
I hope for the best man, good luck