Funny how we can be at the peak of rationality, full of sound advice for our friends, and never "that person" that turns into a pathetic puddle of a woman until you meet somebody and it happens to you. I think part of you that doesn't want to let go is because he's hanging with somebody else, and it makes you feel like you aren't worth much and a need to prove yourself to be a better partner so that he will choose you and fall in love, and change and be this wonderful guy that you wanted him to be from the get go. That unfortunately is a fantasy. The reality is that he gets away with so much, and no matter how much shit you give him, if you are still there at the end of the day, guess what? He won't learn his lesson.
You are only worth as much as you are allowed to be treated. I say this as a guy that his treated his girlfriends not very well. I got in the endless cycle of staying with them but giving them subhuman care, figuring "well I don't want to crush them by leaving them" and "if I fall in love and meet the right one, everything will be perfect". Well not leaving them even when my feelings are gone is the absolutely worse, most painful way to go, and very selfish because I get to have her when I want to and she feels "powerless" about her feelings and can't leave. And it doesn't matter who I meet, if I don't know how to take care of somebody, how to treat somebody, love is not going to magically turn me around and fix my ailing problems. It seems for some, when they fall in love, they are great boyfriends and do all the right things, although they have problems like everybody else. I've never been in that position with a girl so it's tough to say if love does give you a little extra effort. But no matter how much you love somebody, if you don't even know how to do the right things, love won't make you a good boyfriend. If you are possessive, you will still be that way for example.
Anyway, it doesn't sound like he cares enough and has any respect for you. Breaking trust early on made things horrible for you, and when you can't trust them, it turns into a nightmare. You spend so much time focusing on "are they lying to me, are they sneaking behind me, are they being faithful?" everything around you suffers. You have less energy and less focus for work, school, friends, and even stuff that makes you happy. Meanwhile you are unhappy to have to babysit your boyfriend and take him on like he is liability and an extra responsibility. I'm guessing romance is nonexistant. I will emphasize that you AREN'T HAPPY.
So why do you stay? Why do you give him other chances? The hurt is so fresh and your decision to stay with him was done so hastily that you didn't even figure out what the hell he was doing with the married woman in the first place, and the new details that surface are pissing you off. If you are trying to work things out again, you have to let the past go. Which it sounds like an impossibility at the moment. Your foundation for a relationship is in utter ruins, can you really build off these pieces?
You can't help how you feel but you know it's bad for you. Why do you continue to do it? You are completely responsible for staying in this. You have nothing to work with here and there is too much hurt to make anything good out of it. You guys are supposed to be having fun, that's the first step before you get more serious. Fun? Nope, hardly. You will be okay, he is not the only guy in the world for you. And if he isn't enhancing your life's experience, he's taking away from it.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.