Ummm....both myself and my brother have been called that at one time or another. Truthfully, who hasn't?
Think context rather than the actual definition of the word alone. Work on not taking such a word so literally.
In all fairness to yourself, I think you need to deal with the reality of how commonly people can incorrectly use words to define
their disagreement of the opinions of others. That no matter who you have interacted with, that they are not delivering a perceivable or professional clinical evaluation of your words and actions. That in fact, they are simply using the word "retarded" as a euphemism to define what they either don't understand, or vehemently disagree with in a socially polarized atmosphere. No more, no less.
Perhaps it's best to simply avoid or limit your exposure to
polarized social atmospheres and subjects where people might be more prone to using such words in the first place. And no, it's
not a professional opinion. Just a suggestion of one person on the spectrum to another.