Step Mother Advice

Hi, I just found out my fiancé cheated and has another baby out there. I'm still debating about whether to stay or leave.  I told him the only way I would make it work with him is if I can forgive him.  I still haven't and we need to go to counseling to figure this out.  I never wanted to be a Step Mom but if I stay I will become one and this child will have a sibling soon as I'm pregnant now.  
How do you become a good step mom? What have you done to help this relationship develop?  The child is less than 1 believe.  My biggest fear is that the child will hate me or the baby mother will have issues with me.  I don't have the best feeling about her. Chances are the child may end up with him.  My friends have told me about their struggles with their husbands baby's mama and it scares the crap out of me. It doesn't help me that my fiancé is not that close with his step mom and hated his ex step dad.