Boyfriend texting another girl?

So my boyfriend and I have been together 8 months. We have clicked from the beginning. We had a few disagreements in the past but have worked through it and moved on. He went out of town to visit a cousin for about 4 days. At his cousins house, they also invited their childhood friend (whose a girl). Ever since he got back he has been super distant to me. Then one day he randomly was normal again. I went over and he's phone was there and I went through it (which I never did before but I felt like something was wrong.) I later figured out that he texted the girl that was there with him and his cousin. Saying how hot he thinks she looks and how delicious her butt looks and etc. I confronted him but not till I got home. I told him what I saw and whatever and he become super apologetic and said he doesn't know why he sent that message and he was at fault. I asked if they hooked up and he said no he didn't hook up with here on the weekend but has hooked up with her in the past before I met him. He also said he doesn't know why he's been behaving so distant lately. I love this guy and don't want to leave him. But at the same time I don't want to be and idiot and stay with a guy whose basically cheating with me over text. He said he will never text/ meet that girl again but at the same time I'm not really sure if I trust him now. I told him we will work through it, and that I don't completely trust him which he will have to earn back. If you guys were in my position what would you do?