Many young people struggle as they try to understand their identity and it can manifest as anger, depression or secrecy among other things. Our parent stories often mention how unhappy or difficult a child was before coming out. With ADHD you have an added challenge. We have some resources on our site (look under transgender) but please also visit https://www.genderspectrum.org This site focused on transgender primarily and has articles about development, parenting, and research that may be helpful to you.
Another thought, consider a therapist for yourself as well. You have many things to deal with that might go better with support. If you don’t find a local resource you might find an option on http://www.betterhelp.com or other online counseling groups.
Best wishes to you and your family,