Should I leave him?? HELP!

Okay so I've been talking to this guy for over 2 years, we met through a common interest. Now he's older than me (part of the reason we have an issue). So when we met we saw each other really often around other people we flirted all the time and from the moment I met him I wanted him and from then on made it my mission to make him mine. So summer was coming to an end and I still hadn't gotten his number. One lucky day we were alone and he gave me his card to contact him. So it started out small conversation and eventually we would text a lot, calls got super long, and he added me on snapchat so things were going well. He knew I was a virgin and always asked me if I even wanted him to take my virginity (he had been with a lot of girls. Very experienced) I always told him it was fine. So we finally started seeing eachother and things progressed. We finally had sex after about 4-5 months and I was falling for him hard. So he had a girl he was talking to over summer that I knew about but when I would talk about her he told me they were just friends and that he stopped seeing her. So fast forward a bit we were still sleeping with eachother and I got curious one day. I asked him straight up if he was seeing this girl still and he told me no. I believed him. He would always tell me we couldn't have a normal relationship because of my age and he would always distance himself and sometimes go weeks without talking to me. But i didnt care because I fell for him and didnt want to let him go. I would always have moments of I can't do this anymore and we should stop but he would always talk to me and I would go right back. My mistake. So I decided to follow my instincts and find her on social media. Pictures of them appeared together and she was calling him babe and her boyfriend. I flipped out. I texted him telling him how could he and how I trusted him hw terrible he is and never talk to me. I was officially done. I cried my eyes out for days and became really depressed. I loved him and he was just fine. He never texted me so I left it. I had never really gotten over it and we hadn't talked in like 7 months. I would occasionally go and look him up to see what he was doing and just to see his face but I knew he didn't feel the same. So one night my friend and I were hanging out (she had been with me through everything) and I was looking great so we were taking pictures and I had a really sexy one and I wanted to send it as kind of a screw you look at me I look great was she worth it. And I wasn't expecting anything. I let him know that I'm nobodies second choice. So he responded and we started talking again he pulled me right back in we talked about it and he wanted to see me again and he invited me over. I was very hesitant. So I decided I would see him. I would only talk with him I would not sleep with him I won't kiss him. We ended up taking about everything and he apologized and he told me he thought about me a lot and that if I wasn't the age I was that girl wouldn't even be there (she's older than me) he would always tell me I'm exactly his type his perfect girl. And I would believe him. Now she lives with him and she didn't know about us and she never would. She knew of me and who I was and how he felt about me but she never knew we had something. I asked him if he loved her and he said yes and I just couldn't understand how he could be doing this. So I left that night and he kissed me. Just like before I fell into him. We started seeing eachother again having sex but he's still with her. I still love him and I always told myself I would never be with someone who has a SO. So I've been going over when she's not there and I just love being with him and being around him but I know it's bad. I keep hoping that when the day comes when I'm 18 we can be something but I don't know if he'd leave her and why would I want a cheater right. My best friend hates him for what he's done to me and what he's doing right now but I still want him. I NEED ADVICE AND HELP! I don't know what to do I've tried not taking again but I always go back I've also thought about telling the girl but I just do not know. PLEASE HELP!