Update:

My mom got back together with her boyfriend, and my fiancé and I told her that she can't just demand to live with us like that, especially since we have a baby coming. My fiancé told me that he wasn't mad at me, just mad that she constantly takes advantage of me. She started to get snippy and I told her to stop being so ungrateful. In the end, she decided to stay with her boyfriend and look for a place of her own in the meantime.

The real reason why I give her so many chances and try to help her is that I just want a mom. Especially since I'm in such a vulnerable spot emotionally right now. And sometimes I think that if I help her, she'd give me some semblance of being a mother to me. I know it's a fruitless effort, and I'm just digging myself into a hole, but the day my dad died, I felt like I lost two parents. That's a whole other wound that 20 years later, and 4 years of therapy still hasn't healed.

Thank you to all who put your input, I guess my fuck up has been resolved.