I’m stupid 😭

I’m almost 29wks pregnant with my bf of over a year. I have a daughter from a previous relationship. She’s turning 7 this month. Yesterday she had a bad day. She was whinny, she talked back some, didn’t want to listen and just not on her best behavior so he told her when we get home you’re in trouble. I assumed like always I’d handle it. When we get home he grabs a belt and acts like he’s gonna spank her (he’s NEVER done this before and I DONT SPANK my child with a BELT) so I rush over and I’m upset telling him he’s not gonna spank her with a belt. He gets mad and hits me really hard on the leg with a belt. It hurt bad so I flip out and start yelling. I send my child to her room so she doesn’t have to see this I told her we need to have an adult conversation so she leaves to her room. I can’t remember all that was said but I’m so heartbroken and my leg is bruised and a little swollen in that spot. He’s never done it before. I wanna leave because it’s unacceptable behavior for my daughter to witness but I love him and we’re having a baby. I’m so depressed.