I’m ruining my relationship

So I’m a 21 year old mother with a very fucked up dysfunctional family. My past has really screwed me up, I had a daughter (my second child) in 2015 and she passed away due to heart issues, my attitude is horrible towards the ones I love, I’m always in defense mode, I have very very low self esteem. My upbringing was completely fucking insane. My fiancé doesnt want me to work, I stay home with my four year old son and his 5 year old son. I feel pathetic and worthless, I don’t know how to be a mom I guess.. my parents were shit and I didn’t really learn anything from them. I try to punish my child but I can never seem to stick with it, I guess you could say I get lazy with it. But please believe me I am trying, idk why I’m like this and don’t know how to help my own child. And my fiancé practically crucifies me for it, he sees that I’m trying but it’s still not good enough, his son acts like an angel compared to mine and it’s because they had different upbringings, we’ve been together for a year and a half now and he and my son but heads like no other. He treats his son differently than mine, he still punishes him for his wrongs but there’s more of a father son relationship there. Now I get that that’s his son and what not but he came into this relationship knowing that my son had no father and he said he would be there for him. But isn’t there like he is with mine. My son does have an attitude and doesn’t like to listen to my fiancé and I could understand why that would upset him but when my son does something or isn’t paying attention to my fiancé as he’s talking he walks away and leaves my son there, and at times I don’t even know what to say to my son that’s why I feel so pathetic as a mother and women. Now if that were his son he would sit his son down and talk face to face with him about the problem. I understand that my son is my responsibility and I should be the one doing every thing but sometimes I don’t know what to do :( i hate myself for the mother and person I am. We fight a lot over the kids, money, my stupid family. I don’t know how to stop the fighting over stupid shit. I don’t know how to be a. Better mother, step mother, and fiancé. I need help and advice. He’s close to ending it and as well am I. I don’t know how to fix our issues and make us better as a couple. I’m sick of hating myself for not being able to change. Please any advice will help.... I don’t want my family to come to an end 💔😭😓😰