I'm not sure if I've messed up in my relationship and I would appreciate some advice on what I should do next.

I've been with this guy for four months, I like him a lot and I'm definitely in love with him. We get along really well and we spend a lot of time together, we really enjoy each other's company. Our sex life is really good, fun, active (not sure what more to say on that). We have never had a big fight and rarely argue or disagree. Our relationship is peaceful, loving, fun. It's pretty much everything I've ever wanted. He's smart, attractive, interesting, a good leader, and responsible. I've always felt comfortable and secure with him too. I honestly cannot think of any major issues other than the reason I'm posting.

We've never had a conversation about commitment or being exclusive. He's only my second boyfriend (and sexual partner) and I can admit I'm kind of naive and shy in general. I'm not an assertive person at all and I avoided the topic despite wanting to talk to him about it. I figured everything was fine because I was happy, he's happy, we see each other a lot, etc. I've heard you're supposed to have a commitment before sex but I had sex with him anyway (about a month after our first date). I know I should have waited. I should have been more assertive about commitment but I was really into him and it was just too hard to wait any longer and it just happened. I come from a religious background, so trust me I feel bad enough about having two sex partners and sleeping with my current boyfriend so quickly (and without being in an official relationship). I definitely feel like I deserve the anxiety I'm having now.

On Thursday he was with a bunch of people (at a bar where my friend's bf works) and left with a girl he'd been hanging around and flirting with all night. I didn't hear from him that night which is unusual but I figured he was tired and drunk. I didn't think anything of it until my friend called me yesterday and said her boyfriend saw him.

I know I have no logical reason to be mad or hurt because we've never talked about being exclusive. He introduces me as his girlfriend, between that and my own delusions (?) I guess I thought we didn't need to talk about it and he was being monogamous. He has jokingly (I think?) asked if I was with/fucking anyone else but assuming we were exclusive I took it as a joke, answered "of course not" (the truth), and never thought to ask him.

The problem is I'm hurt. I'm not angry but I'm definitely feeling upset, I've been crying all day. I'm not sure how else to describe it, my heart just hurts. We talked briefly last night and I didn't say anything about it, I told him I wasn't feeling well and got off the phone quickly. He's coming over tonight and I don't know what I should say or do. I feel terrible about myself and I just want to know why he's still seeing other women. I think I'm a good girlfriend. This sounds really narcissistic but I'm pretty, I put effort into my appearance, body, hair, skin. I have a good attitude. I'm agreeable and I'm a nice person in general. I respect him, I listen to him, I follow his advice. I make sure he knows I think highly of him. I get along with his friends and family, I actually really enjoy being around them. I'm not sure what else to include. He inspires me to want to be really good to him. It's extremely important to me that I treat him well and I put effort into it every day. I honestly don't know what else I should be doing or what I've done wrong that's preventing him from committing to me.

I know I messed up by not talking to him about commitment and having sex before I knew we were exclusive because commitment is so important to me. I just don't know what to do now. I want a monogamous relationship but I'm scared to bring it up. I've also read some conflicting information here, some say not to have a "talk" but others have said it's necessary. I guess I'm confused about that. I would like to try to keep the information from my friend/her bf to myself but I'm not sure I can. Should I mention it to him? I'm only mad at myself and I know it isn't his fault I'm hurt, I would make sure to communicate that to him so I don't sound accusatory. If anyone has advice on what I should do I would really appreciate it. FWIW I have talked to my sister about this and she replied "he's selfish and gross, respect yourself and stop seeing him" Not exactly helpful.