I understand, I torpedoed my first attempt at transition due to family matters, then spent the next twenty years feeling miserable. You aren't doing this for them, you are doing it for you. Your happiness is what counts here. You aren't hurting them, their attitude to what you are going through is what is hurting them.
When I saw Lt Col Cate McGregor speak about her experience of transition she said that she perceived transition as an inherently "selfish" thing, but that it wasn't a bad thing to be selfish, that it was the only way she could see to crash through the barriers and excuses against transition other people were throwing up around her. I wish I'd had that attitude the first time around, and still need it now.
You aren't your mother's "son" you are her child. Parents invest a lot of their energy into gendering their children at the expense of seeing them as a person. She isn't losing her child.