Mom venting 😢 I’m lost

I have a 15 month old son and one that’s due in April. Lately my son has become a full on toddler and gets into everything! Fine, no biggie, except my patience with him is wearing thin because literally every time I change his diaper it’s a fight. He screams and cries and there’s nothing I can do to make him happy, and once he’s in that mood it’s impossible to get him out of it. He slapped me in the face 4 times in a row and nothing I did made him stop, I feel like a failure of a mom because my kid doesn’t act this way around his dad. I’m trying really hard to be more patient but the slapping was my breaking point, I just broke down today. And because he doesn’t listen to me at all, it makes it difficult for me to be patient with the things that should not bother me. I just don’t know what to do anymore. I’m exhausted from the fighting with him and trying to get him to understand and listen and pay attention to me.