Boyfriend caught sexting another girl

I've been in a relationship with my boyfriend for 3.5yrs and throughout this I have been in college studying for a degree. Our sex life used to be good but he was always trying to dirty talk with me and I didn't like what he would send or the way he worded things. I did try a bit but with the pressure of college and everything else i stopped but in doing this his sex drive has gone down and he saw it as me trying to suppress his sexuality. I just didn't get it or the need for it. Don't get me wrong I used to love dirty talking but he thinks my dirty talk is extremely tame and gets bored. Recently I went through his phone (yes I know I should not have but I had suspicions) and found he had been dirty talking to another girl. He says he hasn't slept with her or had any oral sex on either side and that it was just dirty talk. We both love each other and want to make it work. But I am so not confident with his dirty talk and have been looking stuff up on line and there are stuff that I do like. My issue is is that I need him to be more caring and attentive and also how do I dirty talk with him when I get embarrassed and feel like an idiot when I try, I just don't know how to start it but really want to spice up our sex life and actually have sex again and learn to trust him. Please help any advice would be gratefully received.