There is not enough information here to know if this would be beneficial. But also the tone and intent would be important. "I wanted to let you know as a learning experience for your future patients" is a much different message than "You misdiagnosed me 20 years ago and screwed up my life and I want you to feel bad." If the only reason you want to do this is "for closure," then I think your idea of confronting your pediatrician is likely misguided because it will not actually bring you any closure. Only you can give yourself closure and move on.