CHEATING OR NOT?

I'm really very confused right now. I've been with this boy five years now. I lost my virginity to him. I love him very much. We're very open with each other. I tell him everything. When I got attracted to another boy I told him and we talked about it. I went to him one time and told him, let's be in an open relationship. He got really upset and mad. Said he didn't want to share me with anybody else. I was the only girl for him. That kind of stuff. So I said fine. Since you said no to it, you can't cheat on me. So I decided to forget about it. The reason why I asked for an open relationship is because;

(A story within a story);

My ex boyfriend, he kind of betrayed me. He got really close with this girl. And when we broke up the first time he dated her. Then he came back to me and said he wanted me back and he was sorry and he wanted a second chance. So I took him back. Then after a year, he broke up with me and went back to that girl. I say he betrayed me because before he first got with her, he said all these nasty things about her. He made me hate her for no apparent reason but just because he was my boyfriend. I hated her so much. Then he went behind my back and dated her. The betrayal really cut deep. After a while I got over it. He broke up with her and said she told him all these lies about me that's how come he broke up with me and he was really sorry. But it was already too late.

(Back to the real story).

So by now I was already jaded and built all these walls. I never let anybody get close enough and somehow I managed to push everybody away. I fell in love with myself because there was no way I was going to let someone else hurt me like that again. But I had this friend,(James) he liked me but we both weren't looking for a relationship. We used to make out and stuff, no sex though. I was virgin. I told him and he said he respected that. So he never forced me to do anything. Then one day he gave my number to his friend.(Robert) I didn't know about this development. So his friend hit me up one day and we became good friends. Went to see him and we had sex. I lost my virginity to him. So then I had this bond with him. For a long while I couldn't figure out why I couldn't stop thinking about him. I thought it was love. Then one day it hit me; he was my first. There was a bond. Anyway, he had already fallen in love with me ( so I guess that was a good thing? I mean, he didn't sleep with me and then just leave.) So we became really close. Then I found out a year later that the whole thing was planned. James gave my number to Robert because he didn't believe I was a virgin. So Robert was supposed to sleep with me and then leave. That was the plan. But he ruined it by falling in love with me. So after I found out, I was really hurt. I deleted them both. But they came back and said sorry. I really couldn't let Robert go because he was my first. It was so hard. James did everything to ruin it because he was mad at me for choosing his friend over him.

Long story short, I stayed with Robert, he never asked me out or anything, but he told his friends I'm his girlfriend and introduced me to his family as his girlfriend.

On 20th of March 2017 he told me about this girl he had feelings for. He said he caught feelings for her in 2015. And it died that same year. I was confused for a bit because he was just now telling me. So he came over on the 25th to spend the weekend. And on the 26th when he was sleeping I checked his phone.(That was really, really, really, really wrong I know. It's not me. I'm not that girl. But I got really curious about her.) So I checked his messages with her and what I saw really broke me. He told me his feelings for her died in 2015. But I saw messages from 2016 to 2017. Saying how he wants to kiss her and have sex with her and stuff. So many things. He woke up when I was reading the messages. And he said there was nothing going on. But I don't trust him. He said he wants another chance but I can't get all those messages out of my head. What should I do? I just don't understand why he said no to being in a open relationship and then cheated. He says he wants another chance. I'm so confused.