Should i break up with her or forgive her?

My girlfriend told me wen we weren't together that she went to a party got drunk and started making out wit a random guy and went to his truck she said all they did was make out and that's it. She said she did that because she was mad and hurt that I ignored her and she thought I didn't like her nomore so she didn't care , her and me have some history and I broke up with her cus it was to much fightin, but we got back together like a week ago but she made out with that guy a week before we got back together,so it was recently,ill understand if it was long time ago but it wasn't. But even tho she wasn't with me wen that happen,it still hurts picturin her wit someone else like that. She said she didn't like him or anythin she just did it cus she really didn't care,she said she was sorry a million times,she was even beggin me to stay with her. So should I stay with her or leave her?