As the others have told you, Open adoption is what you make it. It is a decision you & your partner make. SOmething you have to be TOTALLY comfortable with!!! It is also something you have to be honest with the Birth Mom. We have 3 very open adoptions. They include frequent visits. We told our childrens Birth Mom basically, "This is the MINIMUM we can give you as far as an open adoption. If it goes further than that, GREAT, if it doesn't, this is the minimum." Open adoptions also change over time. All 3 of ours are so different. Lives change, people change, but our basic promises in our relationship never does!
As far as being the MOM, our children know who MOM is. They know whose tummy they grew in, too. "I" am theone who has a tug at the heart when our children are talking to their BMoms on the phone & then my child will say, "Do you want to talk to Mommy now?" I feel a little pain for them.
Open adoption takes time to develope. Just like in ANY relationship, it takes time to grow. To trust! When we adopted our children, we just didn't adopt a child...we added a WHOLE extension to our family! Our children have SOOOO many Aunts, Uncles & grandparents! LOL All of our children call all the others Grandma & Papa, too! They have 6 sets of GRANDPARENTS! LOL With our son, he has his siblings stay with us often. Over the 4th of July week we had all 4 of his siblings stay for the whole week! (We don't have any contact with any of the Birth Dads.

LONG story for each.)
Just be honest with youreslf & your PBMom. Do some more research, talk to more people. When we set out on our adoption journey, we were somewhat like you. Now, I couldn't imagine our lives without the kind of relationship we have with our children's Birth Moms!
GOOD LUCK!!
Deb