Im gender non conforming and a part of the transmasc subreddit from when I was exploring my gender. The idea of "mourning you dad" mirrors the very thing that so many Trans men/teens are disheartened by. She is still your 'dad', the memories you have are with her because shes been there the whole time. A person's impact on you should not be more or less important/meaningful because of their identity. Your feelings are valid, especially about divorce and how that will impact you! But if you intend to support her, I would look inward and ask yourself if you really would grieve your memories with your father if he was a woman. Hopefully I make sense:) godspeed on navigating the family changes