The birth parent has NO obligation to the child after providing for them--in other words, the birthparent gave the child to loving adoptive parents, who made sure the child had a place to sleep, enough to eat, and all that jazz.
While it would be nice if it always worked out that they could be friends later in life, I can think of several reasons a birthmother would walk away and not look back.
Think of all the traumas that could result in an unplanned pregnancy. What if it were rape? What if it were abuse? What if her parents disowned her for getting pregnant? So many "what ifs" that could have been VERY bad memories and times for her. She could have moved past all of that and had a normal life--but ONLY because she was able to choose adoption and move on.
You are roughly the same age as my daughter. Open adoption was coming into play at that time, and more and more people were choosing it. There must have been a good reason to NOT choose open adoption, if it had been an option for her.
Look, I'm not trying to destroy your hopes here. Many reunions end up very happy. But there are several that I know of that did NOT end up happy, and the adopted child felt worse in the end than they had to begin with.
Since your adoptive mother refuses to discuss this with you, I have a feeling that she knows something of the story behind this, and is trying to save you some hurt.
I'm not telling you not to look. I'm telling you that there are valid reasons for NOT wanting contact with a child you placed for adoption. I'm telling you that not all reunions are happy, and that you shouldn't go into it expecting to find a great friend.
I STILL suggest counseling for you, to prepare you for either possibility.
And I would hope that if your birthmother refuses contact with you that you would respect that and move on with YOUR life.