What should I do?

I have been in a relationship for 2 almost 3 years now I have a son and a newborn daughter and a stepdaughter although we're not married but anyways I thought our relationship was going good never once did I ever questioned anything until about 5 months ago when I was still currently pregnant I was six and a half months along at this time I felt something was wrong so I decided to do a little snooping and come to find out my fiance has been chatting with other girls practically since the very beginning of our relationship all the way up until the point that I caught him he was saying he wanted to meet wanted his d" sucked or do you wanted to meet up for sex kind of things to these girls messaging them and chatting with them through a Text Now app and also had a email and a Facebook account under another guy's name that he also used to communicate with these girls and he was on numerous dating sites as well chatting with other women according to him he's never met up with any of them for anything physical it's just chat and he doesn't even know why he did it kind of thing but the way he was talking to these girls about meeting up sounds to me like he probably already has cheated physically he just don't want to admit that part so he just says he just like to look at escort Pages just cuz he just like to look at it or he was just chatting with these girls just to chat with them but anyways finding this out when I was pregnant with our daughter just broke my heart and honestly I thought our family life was good we have sex all the time so I really didn't think that anything was wrong and not once did I ever question him until I found all of that on his phone well anyways he told me he would stop he wanted to change he loved me and her family and that after I gave birth to our daughter that he love me even more but now I came to find out he's still been going to escort pages and still has a different Snapchat and everything else so now I have two kids and he's the one who pays for the majority for the house so I'm going to be pretty much left with nothing and I am scared that I am not going to have anywhere to go with two kids what do you guys think I should do about this should I just leave and not look back get a job and move out should I have him do a lie detector test to prove he hasn't ever done anything physical during our relationship or do you guys think the sneakiness of him practically doing this our whole entire relationship is enough to just end things and move on because otherwise he would just think that he can just get away with it and keep breaking my heart and I don't want to waste more time for me or the kids but the kids will be devastated because my son is 3 and my stepdaughters 4 they've become really close with each other and I think in the end the kids will be the ones hurt the most please no harsh comments I just want some advice! What do you ladies think or have you guys delt with this same situation?

Get counseling. You need to learn to love and respect yourself before you can hope to be in a healthy relationship. He has cheated from day 1 and you knew it but looked the other way.
And to make matters worse you had a child. He was and never will commit or marry you. Move on and next time don’t give everything to someone who has not earned it