It's not as hard as it seems.
What if everyone was accepting towards each others' differences?
Sounds crazy right? Impossible? Only if you think it is. Start small; if you know someone with a mental disability, don't be afraid to be kind to them! That autistic kid that everyone avoids comes home and cries everyday. That girl with Down Syndrome will think you are amazing no matter what you say about her. What's the point of being awful to someone so wonderful? All people should be accepting and tolerant towards others' disabilities.
What you say matters.
According to nih.gov, "Negative behaviors and media images of people with mental disability impair self-esteem and help-seeking behavior." We're all humans, so there is no need to be negative towards anyone.
Also, as stated on aucd.org, "People with mental disabilities say that negative language leads to harmful action, abuse, discrimination, negative stereotypes, disfranchisement, and violence. You should understand that if someone harms them self because of something you did, it's not on them, it's on you.
Try saying kind words, instead of being unnecessarily rude. Did you know that simply saying "Hi," to a special-needs kid can make their day? A long time ago, Aesop said, "No act of kindness, no matter how small, is ever wasted." Even if you only say a single kind word, it will make a big difference.