tourist visa to immigrant visa help

[ion21]

hi here's my case.. I was adopted by an american citizen, the adoption was approved on the year 1997, i was 13yrs. old back then. My step father who was a US Citizen died last 2007 failed to apply for my petition to be an immigrant here in the US. according to what i have read my mother could apply to be an immigrant as well as for the child "IF" the child hasn't reach the age of 18. my problem is that I am 25yrs. old now. I was granted with a tourist visa but wished to change it into an immigrant visa. we already consulted the US embassy in my homeland but could not take any actions regarding this matter because according to them I have a different case, i was told to ask the immigration here.

Just want to know if there's any possibilities? if so what are the requirements, etc.? hope to hear from you regarding this matter.. Thank You and God Bless!