Thanks for the welcome, zoo and runnerduck, and I'm sorry that you've both had a tough morning. It started off rough here too, although we did manage to get out for a walk, which has helped.
I have a DH, who is great. DD is an only. The guilt is pretty huge - DD got her diagnosis aged 10, which was purely down to me pushing for it (rubbish primary school were not bothered because she was no trouble and doing well academically). But that was 10 years of parenting an autistic kid like she was neurotypical, which I think has probably been fairly damaging. Thanks for the idea of private psychiatry - when I think back over the last few years, there have been very very few times that she has shown any joy in life. She hasn't wanted to do anything for at least 18 months, maybe more. So it feels like her depression is clinical rather than reactive, IYKWIM. I'll look into EHCP too. She gets DLA - might be worth us applying to up that a bit. She also has a very wealthy granny who will pay for anything she needs (Granny doesn't buy things that people WANT, but is super generous with money when it comes to need).
We've had no support in terms of parenting an autistic kid. But I have learned LOADS from the autistic community. There are some amazing FB groups - it's very very uncomfortable at first - the groups are autistic spaces, and it is a real mental shift to stop viewing autism as a disability and an issue, and start seeing the neurotypical expectations as being the main problem. But there are so many autistic adults who are absolutely brilliant at thinking of strategies and recommending supports.
Bra-wise - will she actually wear one? I just ordered a shed-load off the internet (we found Boux Ave to be good because DD is titchy, and they do good non-underwired bras with small bands and larger cup sizes). DD tried them on and said which one fitted best. I just ordered a couple in that size. We also tried Molke as an alternative, but they involved a bit too much wrestling to get on. DD hasn't struggled too much with periods, but period pants have been good. We don't bother rinsing etc, just bung them in the wash fairly quickly. Not had any issues!