I sort of understand you... It's not fun and my mom asks me the same thing, but I think it's just how mothers are...they see everything that we get done as unnecessary. First because they usually think we're fine the way we are, that we don't need the changes... My mom kept asking me in the past if I really needed SRS to be happy, that I could find a guy that likes me without it, because it is such a harsh procedure and they just don't see the need, even if we tell them, there's a certain limitation in their minds. They want the best for us, they want us to be sure of the decisions we make, cuz the surgeries we get make a pretty big change, they make us go from one sex to the other, I don't think there are more extreme results in any other type of surgery. So it's a pretty big step, one that will give us very little to no chance of return and as our moms are not trans, they don't fully grasp why we need them...
Plus, moms have the protection factor, they have a need to ask us the "are you sure?" question so they feel like they did their part on protecting their kid. If they don't ask and something goes wrong, even if it's just the surgery that didn't go right, they will blame themselves for the rest of their lives for not even trying to protect their kid.
I also get bugged by it sometimes but every time she starts with the questioning I sit down and tell her how much of a difference it'll make in my life, and I try to make her see things through my perspective, how I feel and how many issues could be solved with that decision... I still live with mine so she sees my struggles and the things I go through that, had I had the surgery, I wouldn't have to deal with anymore. As they're not in our heads, unless we make them see things through our perspective, they just won't get it. And it's important to remember that they're not trans, so we'll have to have this talk more than once cuz, as time passes, if we don't remind them, they'll start to forget cuz it's not how they think and see things. Specially if you don't live with your mom, then your struggles, how you feel and think are all far from her day to day life...
If she's supportive it's really all that matters, the questions will go away eventually once she sees you're happier with the changes