Not to be offensive, but your mom is a selfish woman. Children often hurt their parents feelings. Just as parents hurt our feelings. We are human. Then we forgive them and keep going. We don't throw away a relationship unless the adult child is a really horrible person. But even then we rarely stop loving them.

As a birthmom I am very offended that an adoptive mom would do this.

Why? Becasue she was supposed to be a better mother then the birthmother.
This kind of action is not what any one would expect.

You dear child! Like most children, adopted or not, you are taking responsibility for your mother's emotional baggage. Her anger or whatever negative feelings she has are her problem. You are the child, the one who is to receive support and love, and while all of us can think of things we could have done different to show love, children are not supposed to provide support for their parent's emotions. Since she has abandoned you, your life is no doubt difficult, but not impossible.
I have seen a similar situation in our family. My sister-in-law adopted a premie baby boy with a cleft palate when his birth mother failed to bond and gave him up for adoption. Twelve years later she went through a divorce leaving her only the boy to support. Twenty years later, the adopted boy began using chemicals such as alcohol, and marijuana resulting in a car wreck and minor injuries. He failed to learn how to manage money and overdrew his bank account so much that he can't have a bank account again. Plus he married a girl that the mother did not approve of. She, my sister-in-law, moved thousands of miles away for a year till after her grandson was born. It's a long story, but the boy and his wife lost custody of their baby and my husband and I stepped in and were given custody when he was 2 months old. Now he is three and we hope to adopt him. The grandmother seldom sees our great nephew, and I feel it is her loss. I am enjoying having another baby in our house! But then he is my tenth child. The others are all my natural children and love him dearly.
YOur adopted mother is the loser. She is missing out on seeing her daughter grow into a mature woman. I have seen natural daughters abandoned as well as adopted ones. So being natural child doesn't insure against being hated.
Your future is still to be written, the past doesn't have to be repeated if you choose to find friends that are good for you and seek counseling to deal with the grief of loss. It really helps!
I will be praying for your success in growing up whole, emotionally and spiritually. There is a Psalm that says "When your father and mother forsake you, then the Lord will take you up." I believe that God provides for loving friends if we seek for and accept them. ML