Mother has had unofficial custody of 2.5 year old since 8 months can she contest adoption?

Mother has had unofficial custody since my child was 8 months, who is now 28 months. This was voluntary. I’d like to place my child for adoption. Father is not in picture and unable to be located. My mother says she’ll take it to court and “have it tied up for years”, and that she’ll win. But I have legal custody. I am the legal parent therefore I can choose adoption right? Can my mother contest it and is she likely to win? This will likely result in loss of most of my family and friends, if I place my two year old whom they love but I feel that my child will have a difficult life if I don’t. What are my next steps if I choose to place?