You're correct that once the child has turned 18, the child will be an independent adult capable of making their own decisions. However if the child, now adult at 18, cannot provide for him or herself, the adopted parent must apply for guardianship. Courts will usually not dispute the transformation into guardianship for disabled persons, unless a person raises an issue with the guardianship. You should raise this issue at court, and try to establish some birth parental rights there.
Alternatively, what sort of adoption process did you have? There are actually different kinds of adoptions, closed adoption with no contact from birth parent, limited contact adoption, with exchanges of letters with birth parents or open adoption, where children and birth parents do interact. I don't know what the adoption papers are, but you might want to ask to see them. If they were open or limited contact adoption papers, then the adopted mother is illegally depriving you of your rights. If the adoption is closed though, you might have to fight your way through court.
Please find a family law attorney for legal advice. Best of luck.
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