You have to do all the work yourself and then some. Sucks, but that's how people are. Like this:

Mom, I've got something very serious and important to talk to you about, and it may be a difficult conversation. Would you have time at some point to please sit with me for an hour or so and have a discussion?

[later]

Mom, I'm a boy, and I need your help. This may be hard to understand, and honestly, it's not easy for me to understand, but the sense that I need to be boy to be a whole person is not going away and will determine the course of my entire life. Please help me with [a specific ask, like: acceptance and understanding, professional guidance, emergency mental health care]

(There's more stuff you have to do, like remain totally calm and take breaks if you get emotional, take breaks if the other person gets emotional, and revisit the conversation after giving her time to process.)