His in office cheating thing!!?

3 weeks ago I found texts between my live in boyfriend of 4 years and a women who works in his office.
This isn't the first time he was having a "text flirtation" the worst part is that he bad mouths me, instead of just not saying anything about me he say that I'm mean and that he can't wait to leave me etc.
So I confronted him about the texts and he says that this thing just happened that morning and nothing happened it was going on for a day...A week later I decided to check his work phone that he keeps in his work car.
That phone also had a bunch of texts even dirtier then the last ones.
I woke him up and couldn't stop shouting, he then told me it had been going on for a month but again nothing physical had or occurred even though he had planned to help her move out of her apartment behind my back.
I called her up botched her out because she knew that he wasn't single in fact this skank has hit on my father (also works there) I let her have it.
My boyfriends excuse was that she was nice to him and his self esteem was none existent blah blah blah.
I love him but this is the only thing on my mind now whenever he goes to work I don't know how to forgive him or how he can gain my trust.
His in office cheating thing!!?
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