As a person with a writing disability, what’s the best way to respond to people who insult my writing then my disability?
First off, your question is formed well, so there appears to be no writing disability there. You have a good way of communicating your issue, so maybe it is not your formation of thoughts that is the issue, but how you plan what you are writing.
How do you react? Well, understand that there are a lot of people out there who will always try to pull you down. It does not matter what they can do themselves, they just want to feel better by kicking you.
A while ago I wrote an article on how to deal with negativity from readers. It covered mostly a derogatory rating on Goodreads, but it can also be used here. When a person comments that they had difficulty with your work, take it more as a constructive criticism than anything else. It could be just to bring your awareness to something. There is no need to tell them about your writing disability or argue with them. Remember, if it is a book or paper and they paid for it, that person is a customer. The last thing you need is to argue with them, for you will lose.
Challenging yourself in writing, even with a writing disability, is very commendable. In order to make sure your writing is as good as it can be, I would suggest always having someone look it over before sending it out. In the question of books, employing critique groups helps clean up some issues, and a good editor will catch the rest. When all else fails, read the work out loud to help find the issues.
Never give up! Even with a writing disability, you will find by continuing to write you will overcome the issues and improve over time.
Good luck with your writing.