Postpartum hormones or something more?

I don’t know if it’s the stress about work, having a 4 month old, or just hormones but my relationship with my fiancé is falling apart. At least in my eyes. A little back ground. We’ve been together for three years. We got engaged two years ago we were going to get married November of 2018, I got pregnant and gave birth August of 2018. Our relationship has been kinda tough always, he’s 6 years older than me and we’ve been dating since I was 18. So I’ve never really been able to go out or anything with him, this has always created an argument because he’s always chosen to spend time out with his friends vs me. Well when I was 8 weeks pregnant he told me he had cheated on me the previous summer, a one night stand deal. I stayed if he agreed on some conditions and we go to counseling. We have yet to go to counseling, we don’t have the time or the money. But I will say when I was pregnant he was great, and so was our relationship. After our baby was born it was great until I went back to work. Since I’ve been back to work it’s been hell. My mother watches her and sometimes he’ll get off work and go home to play video games and take a nap before getting our baby. He never communicates with my mother, we do not pay my mom and she does this out of the goodness in her heart. I feel like this is not only disrespectful to my mom but our baby. For his birthday his friends invited him out, not me for some reason they never invite me, even though they are going somewhere I could go. He asked if he could go and I felt like I couldn’t say no because it’s his birthday. But now I have this feeling like it’s wrong. Why would he want to spend his birthday with his friends and not his family? I couldn’t imagine. My life is so upside down and changing so much and he seems like nothings changed for him. I don’t know if it’s just me but I’m starting to question everything. We are planning on getting married and buying a house this year and I’m scared that it might not be a good idea. I’m worried he might just be staying around because it’s what he’s supposed to do and he’s comfortable, not because it’s what he wants. At least that’s how he acts. Holding it together for my baby but man it’s hard.