Don't hate me. Opinions please.

So I feel really bad but I must admit that I just absolutely can't stand my future step son. My fiance and I have been together for almost a year and a half. I've know since I first met him that I was going to have a problem. My fiance is a single dad. His Bm left him with the child. He is 6years old. Since I found out I was pregnant, I'm 13weeks 3days it has became worse dealing with the son, everything he does annoy me, my eye starts to twitch and I say stuff to my friend like I need to just separate myself from the home so my baby doesn't look like him. I understand he is just a boy and I'm really trying to be patient. Any suggestions on to what can help?