Feeling defeated

I have spent years fighting for my marriage, we separated for a year then came back together. I was very clear that I wanted a monogamous relationship to sort through our issues. He was in agreement, but later on has admitted he wants to be polyamorous. I told him that if this is the case, then I don’t want to be in a relationship anymore. He said ok, that he’d rather be with me than not at all. I am not dumb though, I figured this would come back around again and it wouldn’t be so simple. This time I have found a secret social media account where he posts provocative pictures and communicates with other people online. At this point, I’m not even hurt, just starting to realize that our relationship seems doomed and I’m not happy. There are things I love about him and I have wanted to fight for us, but I also deserve happiness and to be loved in a respectful way. For me, I just don’t understand how it can be so difficult to be faithful to one person that you love especially when you are trying to build a stronger foundation of marriage. I even told him that I wasn’t opposed to polyamory down the line, as long as we were steady and strong. All of this is even more so amplified as we live in his parents basement while we save to buy a home, we have a 4 year old daughter, he doesn’t work or contribute financially, and he sleeps in until 12:30 or 1 every day. He does help with cleaning. I know marriages can be like businesses, so even if we didn’t make sense romantically, if we made sense financially and worked well living together, it could be something worth saving. I’m just frustrated, I wish he respected me and I wish that I didn’t have to make the choice of ending things.