Help With My Daughter?

She was born a boy (MTF) but she came out to me a few weeks ago (he is 16). I told him that I accepted her completely and that I loved her. But I'm just having a hard time calling her my daughter. Every time I accidental call her him or son she always corrects me. I'm just having a really hard tim calling her a girl. She was born my little son and was my only son and now it's just really hard. I completely accept her as a girl, but I'm just having a hard time with the whole thing. What should I do?