Step moms and Moms

Hi! 
Just wondering, step moms: how do you feel as a step mom? What are your thoughts on being with someone who has a child with someone else? Is it hard for you? Especially if you're ttc, or your SO has decided "no more children"? 
And moms: how do you feel about your child's father being with another woman? Does it upset you? Do you have any fears? 
I am a step mother to a 6 year old girl, she's absolutely beautiful and I love her dearly. But I've had some issues with her mother as the little one decided to call me mommy and wanted to be with us all of the time basically. Her mother tries to get her to tell my husband to think of her and such. - when she actually has another daughter with another guy already. 
Anywho, I always have a fear that my SD's mother will try and pry her way back into my husbands life. But I know he doesn't want to be with her. 
He also has told me that he didn't want any other children before. 
It sort of made me feel like she would be the only one who ever got to experience a family with him. Makes me jealous actually. 
We were ttc recently, but stopped because he's nervous for what could happen should we "split". And so we could finish school to have our lives in order for a family.