I need to vent for a minute and please give me options… just keep in mind this may upset a few but I am in no way shape or form trying to be a bad person.

Okay, to explain my situation a little, I just moved in with my boyfriend over the weekend, we have been dating going on 7 months now. I have 3 kids, 8 , 5 and 1. Who all for the most part very well behaved. ( kids are kids) well he also has 3 himself. Aged 13, 14 and 4. Guys I’m STRUGGLING and I’ll go ahead and say I feel like an awful person for what I’m about to share, the teenagers have Been amazing! The 3 year old I feel like I can’t handle. He’s very spoiled, very mean and keeps hitting my one year old daughter, yes I know he’s only 3 BUT I am tired of it already, his dad does try to discipline him when it happens but I feel as if he’s just making him think it’s okay with babying him how he does. I’m not going to allow my daughter to be treated that way, regardless of anything and I’m just really stuck on what to do. The part that has me feeling the lowest is that I am feeling like I cant stand the 3 year old now. 😭 which hurts my heart but I’m being honest and needed to vent as I’m only human. She cannot even simply pick up a toy in this house without him getting mad and SCREAMING MINE and having it snatched from her aggressively. He has pushed her over by pushing her forehead, hit her and twisted her arm so far and I just feel like I cannot handle this. What do I do?