Why the mumblechum especially as you don't know the circumstances.
I would have thought you could nullify it somehow, much like a divorce, so that they can no longer be considered my legal parents. I want my birth mother to resume as my mother symbolically as she has passed away. I also have suspicions she was unduly influenced into the adoption.
There are lots of other things you can do legally, such as setting up your own powers of attorney in case something happens to you which means that your APs have no say in your care or your money. As it stands they are probably next of kin unless you have adult children. You can also make your Will and stipulate what happens after your death.
I think RoomFor's idea about having a symbolic ceremony is also a good idea.
There isn't any legal way to void an adoption, except very soon after it is first finalised (if the birth parents can prove illegalities in the process or that they were coerced into consenting). However you are now many years down the line and your birth mother is dead so it would impossible to do that now. Even if it were possible to do it, it's actually pretty difficult to prove coercion. You would need solid proof of serious psychological pressure/outright blackmail in order to do it.
So the way forwards legally is to find ways of cutting yourself off from your adoptive parents. Make sure you have a will to dictate exactly what happens to all your money and posessions after death. Is there a way to tell the medical profession that you do not wish your parents to be contacted in the event of an emergency, even if they can't reach you husband or children?