I’m the evil step mother

My husband has 3 girls and we are expecting his fourth together. We have quite an age gap (15 years) and this is my first child and most likely my only. I raise his six year old full time despite the hardships that come with it. She gives me hell, genuine hell, and after being together and married a total of 2 years it hasn’t gotten better.

Sometimes she’s mean and tries to kick me in stomach, tear up my daughters side if the room that I’ve been working on, snd steals and ruins my sewing projects for the baby. Other times she’s a sweet angel trying to love on me and feel her sister kick. I have a hard time with the switch flip and I can’t help but feel like I need to protect my daughter from her. I find myself finding excuses constantly for why I don’t want her to feel, hiding the stuff that I got for the baby, feeling as though I’m preparing alone and not getting the full extent of excitement I had built up in my head about this moment in my life.

I’m afraid what’s going to happen when the baby comes. I’m afraid that I’m going to become this raging lunatic that just shuts my step daughter out. At this time I don’t want her anywhere near me or my child. Please tell me it’s hormones...