I have never been through this before, but I know there are lots of girls here who have, and have the answers you are looking for. I just wanted to wish you luck, and say I am here cheering you on!
I had a thought though. When you tell your adoptive mother, try putting on your biggest grin, and give her the biggest hug, and say "After meeting her, it really made me even more grateful that your are my mom. It made me realize just how lucky I have been all along! No one is better than you mom!" I think as long as you reinforce that she is not being replaced, it should help.
Really though, best of luck to you. Keep me updated!