People are not treating you like this because you're retarded, people are treating you like this primarily because you are so aggressive.
Pretty much everyone knows somebody in their life who is a little bit slow, but loves being around them because they are kind or are a hard worker. That more than makes up for it, because they are hard traits to find.
Knowledge of facts is not equivalent to wisdom, and the latter is what people usually mean when they refer to intelligence. Yes, it sucks to be underestimated. Yes, we have all been treated unfairly like that at some points in our lives.
Being treated with disrespect because of your disability does not give you the right to treat everyone with disrespect, and will only serve to isolate you. It is frustrating and hard, but that is the life you were born with and you have to do the best with what you have.