Where do I go from here?

Hello everyone,
I  hope you are all doing well, Ive been really sad latetly, my relationship with my fiancee, youngest daughters father is getting bad, we fight all the time he's become a real selfish irresponsible immature father to his daughter and his step kids who are my children and He complains about everything all the time and he just acts and seems like a miserable jerk who has to make everyone else miserable. Since all my kids have come back home and we BOTH have to be responsible he's turned into this person I really can't stand, plus my 8yr old had ADHD and he doen't understand why she does the things she does and he's always yelling at her instead of talking to her, and it causes us to argue because I don't like how he's always yelling at her. I'm scared to be alone and a single mother again at the same time I'm so sick of his crap, things use to be great with us but not anymore,he won't even be responsible for our baby, like today I had to work 2 hours later than the time I get off and told him he had to pick up our baby from the babysitters and he didn't pick her up he had a bs story that made the babysitter angry with me, and I almost lost her, and I want s to work out but he has to change, I just don't know where to go from here it makes me so sad...