Cheating

I found some texts on my boyfriends phone from 4 other girls last year in about October time, we have been together for three years and live together. two of the girls he was trying to chat up and they pretty much shot him down and two other ones were flirting back and it was pretty clear he had met them I feel so confused and heartbroken. Ryan made a promise to me this new year that he was going to change himself as a person and he knows he hasn’t been the best boyfriend. He doesn’t know I’ve found these messages. I also know he is planning to propose to me but I believe that is something his Mam is pressuring him into. I feel like because of when it was I should perhaps’ dismiss it because of the fact that he says he is going to change his ways but I can’t help but feel absolutely broken and worthless. I dont want to leave him I feel like I don’t deserve this. I just want to know what everyone else would do. I didn’t expect this at all from him and can’t believe how much he would disrespect me. What hurts the most is why is he so desperate to bring a child into this world with me but is immature enough to do something like this?