boyfriend masturbates to facebook..help!

My boyfriend and I had a perfectly healthy relationship. He is 17 and I'm 19, he's still in high school and I'm in college, but I commute. We met my senior year and have been together just over a year. However, about a month ago, my boyfriend revealed that he had a slight fetish (facials). I said I wasn't into it and honestly found it degrading, but he kept pressing the issue and we didn't have sex for about a week because of it. Then, I snooped on his phone. He had a "secret folder" app that he used to keep the private pictures I sent him. I opened it up to find another folder entitled "Jerk" containing over 200 pictures of the same three girls from Facebook and Instagram. They are girls from our high school who have already graduated so he knew them from school but had never talked to any of them. Most of the pictures weren't even sexy pictures, and that's what really bothered me - they were the girls' selfies and pictures of their faces! I immediately freaked out and felt extremely hurt and betrayed and confronted him, and then impulsively broke up with him. I deleted the folder and went home.
We got back together the next day because he said he was sorry and, honestly, I was still in love with him and not ready to cut him out of my life. I knew he had only had the folder for a few days because I'd looked in the app a few days before and it wasn't there, and he promised he wouldn't do it again. He suggested just being friends for a while first but I said no, I wanted to be with him. There were obviously a few problems because my trust was broken but things were going okay. We were having sex any time he wanted, sending him nudes, and I even let him act out his fetish on me even though it makes me uncomfortable, because I thought he wouldn't feel the need to look at them. THEN I made the stupid mistake of looking at his computer history. I saw him looking at one of the girls' FB page and pictures a lot, and then he went directly to porn. I, again, confronted him and we got in a huge fight. The next day we decided to take a "break" which lasted about a week, then he asked me back out and I said yes.
Now we are together and things are better than ever: he has been randomly texting me to tell me I'm beautiful and being very sweet. But I just can't shake the feeling that he is looking at girls photos on social networks. Let me say: it doesn't bother me at all that he looks at porn, and I have told him that. But looking at specific girls he knows is not okay with me. How do I trust again? How do I know he's not betraying me? I am obsessed with these thoughts every time I'm away from him. He lied the first time about stopping. He says it's not about those specific girls when to me it seems like it is, because there's a whole internet full of porn and random naked women (and my nudes) he could look at, yet he looks at them. I know he would never cheat on me, but the fact that he imagines having sex with them just breaks my heart. How do I know he's not thinking of them when we have sex? What makes them so special? They aren't even that cute! I don't know what to do anymore. Things were perfect before this happened, and now I feel like everything is ruined. I'm more insecure now than ever. I have a problem with anxiety already, but this has kept me up almost every night. I've beaten this issue into the ground with him and I feel like I can't bring it up anymore. Someone please help me.