Well, You could explain to them that Demigirl is a gender in the Non-Binary category

You could explain that being a Demi girl is sort of like mostly female but somewhat male to them? Or, since they’re proud of you for being non binary, just explain to them that it’s partially non binary too.

okay well, I think you should probably sit down and explain the situation that might help. It's not the best advice sorry

Sit them down and explain that non-binary person doesn’t have a gender they consider themselves as a they. While you consider yourself a she or a they

Explain. If they don't understand just be happy they accept whatever you are.