I relate to this, though I am not autistic myself. My wife and I certainly worry about whether our own personal challenges are going to impact our children. For example, we are both introverts and having to take a kid to a birthday party, mingle and make small talk with other parents is awful, it ruins the whole weekend. Of course, we still go, but our kids don’t have as many play dates as other kids do. You know what, though? They will be fine. We play board games and video games and read.
All kids have advantages in some areas and gaps in others that they will have to work on as they grow. You can’t teach them everything, and yet they will become fully functional adults anyway. You’re doing a great job taking your kid to therapy and getting him help he can’t get from you, that ~~shows that you love him and can take care of him. Focus on passing on your strengths and not trying to avoid passing on your failings.
I don’t know if it helps, but this is not really a lie, and you shouldn’t feel bad about saying it. You have your own reason for not being able to do something you committed to. Someone else might have a different reason that is equally personal that they don’t want to share. “I forgot and I’m sorry” is a socially acceptable way to take responsibility without sharing specifics and potentially making someone else feel confusion or pity.
You can still work on the “why wasn’t I able to do the thing I felt I needed to do” without worrying about “why wasn’t I honest about my reason”.
Just my two cents though.