You sound brilliant, wonderfully supportive, and extremely kind, OP. Your children are absolutely fortunate to have such a great mum, as much as you feel that you are pouring from an empty cup and are getting nothing back.
I agree with others that it is truly a roll of the dice havingchildren, and nothing in life is guaranteed.
My parents adopted me and my late brother, he passed away from cancer aged 34 and was as different and difficult as you could get. They obviously had no control over the children they adopted, as in their personalities, temperament etc, and obviously we don't share genetics so they couldn't ever know he would be dreadful as a teenager, a drug addict, angry and self destructive.
My point is that they gave everything to raise a child, but it was a thankless task in many ways. And even though he didn't show gratitude, they did know he had a good life, and never stopped loving him or worrying. I know the comparison with you and your children isn't quite the same (and i don't know if my DB had any specific symptoms of autism but he possibly had a lot of inherited mental health issues), but as hard as it all is, it's important to get some ways of feeling like you are doing a brilliant job, and life has given you something to be proud of.
My parents probably felt like their efforts were wasted in some ways, but they can look back with a sense of pride at his life being the best it could ever have been, thanks to them.
You can hopefully say the same, in that your children are getting a lot out of life, in spite of how much it has taken out of you. I think you should be able to feel deeply proud of how you cope, and it's evident you are absolutely doing everything you can to the best of your ability. ❤️
Keep going 💪 ✨️