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My boyfriend admitted that he kissed another girl? - greeting message baby

My friend and I had a good fight and I broke with him. Three days later, we decide things. If he is not careful, I took the phone and started to read your messages. I realized that someone greeting "happy bday baby" and had in their messages messages to a girl says "evil meet you" and sent "Of course I want you, I have threatened my job if not wouldt.
I asked them and they said, happened when we were in trouble a few days earlier and asked him if he kissed her and told her, hugged her.
I do not know what to do or feel. A part of me thinks it happened when we were in trouble, but I can not overcome. This happened a few months, in July, but still can not get more.
He said he knew because he angry and upset.

What do you think kissing another girl while you are fighting / try to fix things?