Inappopriate conversations w other girls - please help

I have been with my fiancee for the past 6 years on and off. The last 3 years have been the hardest yet the most wonderful years of my life. We have two kids and our second one is just 3 months. He's a wonderful father and responsible partner. He goes to work and school and he's basically gone all day. Since my 3 month old has been born and my daughter is only 2 it's been very hard for me to take care of them on my own. I have his parents and mine for help, but I miss him a lot and resent him for not spending time with our kids. even on weekends he sometimes prefers to spend time for himself rather than with our kids. In the past I've caught him having inappropiate conversations with other girls online and via text mesaging. I've even left him twice because I was so hurt. 2 weeks ago I caught yet another conversation with a girl on facebook, it was a very hard day for me with the kids whike he was at work. I ended up leaving him the next day cause I was so hurt that he could do this again!...I confornted him about it and he laughed in my face saying he never cheated. That's all he could say that it was ironic that this was the reason why I was upset?...sometimes i just thinks he fucks with my head making me think that I'm some psycho n I'm really not reading what I'm reading. How can he not understand that this hurts so much?..that while I'm at home taking care of my kids hes out doin I dont know what. The thing is I don't trust him I probably havent trusted him in a long time but decided to come back so many times because we have a family and he's great with our kids. I'm currently at my moms and I don't know what to do. He hasnt asked me to go back which I appreciate cause it gives me time to think. But he is acting like nothing happened and makes me feel worst because I took his kids. I've gone back twice before...I miss him already and my daughter asking for his dad constantly and saying she wants to go home is killing me. I don't know what to do.