Katie, what I have to say on this subject is something a lot of people don't like to hear. I think in situations like this we simply need a lot of patience.
I think we need to understand that people we're close to have an image of us that they've held in their minds for a long, long time, and even if they're perfectly willing to change it, it's simply not something that they can do overnight. It takes them a while, sometimes a long while. I don't think that's their fault. I don't think it's a sign of bad-mindedness (if they're not doing it deliberately). It's just the way it is. It's hard for us, but I think it's something we need to accept.
My son, my sister and my brother are all perfectly accepting and supportive of me. Yet two years after my coming-out, none of them are able to call me by my new name. I spoke recently on this forum about my landlord--again perfectly accepting and supportive, and he's long been using my new name. But today, for the first time in a long time, he slipped up and called me by my old name. On the other hand, he was talking on the phone to somebody and referred to me as "she" and "the lady here".
What we're asking of people is difficult for them, even if on the whole they're supportive, as your mother appears to be. I don't think it does any good for us to push them too much on this. They can find that annoying, they can react negatively. We can actually alienate them, when they are making an honest effort. I've seen that myself. So maybe some sort of gentle reminder on occasion, but I think we need to let them deal with this at their own pace. And I think the better they know us, the closer they are to us, the slower their pace is going to be.
It's something I, like everybody else, find annoying, but I don't know how else to handle it. After all, the main thing we want is their support and acceptance. If they're generally showing us that, then maybe we should be understanding in other regards.
