Perhaps you might try to explain
why those elements of being female-bodied upset you, to help her understand. For me, my body felt completely alien and I couldn't stand the thought that everyone who looked at me saw those horrible things on my chest, or heard that high-pitched voice, and so on... because those features misrepresented me and they caused other people to treat me in a wholly inappropriate way.
I also understand your mother's concern that this is just another obsession: she's probably seen you perseverate before and she thinks this is the same thing. Well, I can tell you this: most trans people, including those who are
not on the autism spectrum, tend to experience a similar surge of 'obsession' as soon as we accept the fact that we need to transition. The time between accepting you're trans and starting your medical transition can be a very difficult period, full of anxiety and depression, because as soon as you decide to transition your need to move forward suddenly accelerates until it becomes pretty much all-encompassing and you feel like you need to do it
yesterday. That's a perfectly normal reaction and it looks similar to autistic perseveration, but it's not quite the same thing, as I suspect you've already noticed.
Here's another fact: obsessive phases tend to last something in the region of six months. You've probably found this yourself, and so has your mother. So you could bring this up with her, remind her how long your 'obsessive phases' normally last, and point out that you've felt this way for X number of years already so it's definitely very different. You could also point out that all those attempts you made at fitting in as a girl were actually phases you were going through at the time as a way to help you cope with what you thought was an impossible situation. After all, you probably dropped each of those attempts after a couple of months so you could try something else, didn't you?
There's a lot of research out there that points towards links between the autism spectrum & gender dysphoria. If you Google 'autism gender dysphoria' and click on the link for scholarly articles at the top, you'll find a lot of research material that might help you, including some truly horrible stuff going back to the 1990s. Research suggests that people on the autism spectrum are far more likely to have gender dysphoria than non-autists.
Your mother is probably just trying to find excuses for your dysphoria, blaming it on the autism so that she can pretend it isn't real or is just a phase you're going through. But it can certainly be very real, and it isn't at all unusual to find trans people who are also somewhere on the spectrum. She's also probably going through the 'bargaining' phase of grief - trying to ask you to wait a bit longer & not rush into hormones is a classic sign of this - and the best thing you can do is to gently but firmly push forward with your plans.
Oh, and I have been diagnosed with Asperger's too. My GIC doctor says he often sees improvements with his AFAB autistic patients once we start T, because our autistic behaviours seem more natural once we pass as male. Make of that what you will, but it's certainly true in my case!
