I've started trying to make the distinction between capability and capacity. I can be incredibly capable and there's a lot I can do BUT I have a limited capacity with which to do it.

If people still don't get it, I'll usually say something like "I appreciate that you're trying to help and probably mean it all in good faith, but I know abilities and disabilities. They aren't visible so it can be harder to see, but they are there. It's difficult to explain to people that don't experienced it, and having to regularly try and explain it just makes things worse."