May last week's "slip of the tongue" be a good lesson for you. Words matter. They can cause great joy, and great hurt. They can stop and start wars. It is never acceptable to laugh at or poke fun at a group of people the way you did.
Perhaps most bothersome is that what you said really was a slip of the tongue - something you said that you immediately realized was wrong, and needed to be taken back. Because that tells me that perhaps in the confines of your own space, when the lights are down, and you are just "you" and not the President of the United States, that you do make these kinds of jokes. People don't slip and say words that aren't part of their everyday language.
Certainly, you understand that your conduct must be beyond reproach. That "I didn't really mean it" isn't an acceptable answer.
A great many people in your country have embraced you as the President primarily for your stance on disabilities. In assuming this position, you have accepted the responsibility of setting a good example for the people of our country. Your words, as well as your actions, will be scrutinized.
I'm sure this is one mistake you won't soon forget. I think that's a good thing.