B.A.
I was a stepdaughter who called my stepmother Mom or Mommy Caryl when I needed to differentiate between my two moms. From very early, when I was 6 on until the present, I am 43, I called her this and referred to her in these ways. As I grew into adulthood saying Mommy Caryl became very cumbersome and inappropriate. Starting to call her Caryl as an adult also felt wrong as it seemed to diminish my love and respect for who she was in my life. I wish very much that we had found a way to make a transition to me calling her Caryl or some other special name so that I did not have this challenge. My Cousin who was also a stepdaughter called her stepmother Ellen from when she was a toddler and this has been much more comfortable for both of them. They clearly love and respect one another and it does not cause pain for my Aunt Ellen to not have been called Mom.
It sounds like you were hurt by the change.I think it would be okay to say "I miss being called Mom, but I can understand that it might be less confusing for you to call me M.". It might open a door for dialogue and also might reduce the guilt your bonus daughter feels as she explores her identity.The less guilt she feels the more her love can come through.