I constantly wonder if my parents are happy with me

I feel awful that my parents ended up with me as their only child.
I’m autistic, low support needs, but still.
They had me a little later than most, and I don’t have any siblings.
I try my best to be a good daughter, but I’m fundamentally flawed and I always will be. It sucks that they only had one kid and she turned out defective. I still have childish hobbies and interests, I live at home, and I graduated university but I’m struggling to find a job that will hire me. In 10 years time, I’ll be responsible for looking after them – they deserve better than that. They’re good people. Would their lives be better if I was never born? They’d definitely be better if they had a normal daughter. I want them to have the best, healthiest lives they can have, and I want them to be content with their legacy when they die. What kind of legacy am I?