Should I just give up and walk away?

I met my boyfriend when I was 19 and he was 21. We welcomed our first child when I was 22, followed by our second when I was 24, and our third when I turned 28. He proposed to me when I was 30, and we had our fourth child when I was 33. I’ll be 34 this year, and we are still not married. Recently, we took a break from our relationship; he felt it was necessary to focus on working through the issues we have, which he described as somewhat toxic. He says my attitude causes problems and that there are many things about me that he finds challenging, but I never voice my concerns about his shortcomings because I love him despite his flaws. Lately, it seems like he’s making excuses and might prefer dating multiple women over committing to our relationship. He even joked about wanting a polyamorous arrangement. While we were on our break, he started talking to someone new and is now behaving like he’s single. He moved back into the house, but we’re not officially together; instead, we’re co-parenting under one roof for the sake of our children. Our youngest is not even a year old, and I’m still dealing with postpartum challenges, which sometimes leaves me feeling down. As he began to socialize and meet new people, I also made a friend, partly because I felt I had to move on. I can’t help but think that if we didn’t have kids, we wouldn’t still be living together. Time is passing, and I’m not sure what to do, especially since he is my first serious relationship and all I’ve ever known. I could really use some advice.