help. he cheated.

my boyfriend of 5 years just admitted he cheated on me.. it was only flirting through text msgs. the thing is we are both really good friends with the girl. she flirted back. he tried sexting with her. she flirted but that's it. how mad should I be. it was very PG flirting on her end. but got pretty PG 13 from him. I don't know what to do. it wasn't just one day. it was weeks. they are both very sorry but I can't forgive so easily. i still love him but I can't trust him.