I thought it was macdonalds my whole life
Why did you think it was spelled Macdonalds?
It's just what I've thought since I was little
Well, there's nothing wrong with that!  Everyone makes mistakes from time to time.  At least now you know better!
I'm only saying this because it's you, if it was a stranger I would hate being called a retard.
Everyone makes mistakes and learns things in their own time. It's important for you to realize that and not call yourself negative names just because of a mistake.  No one else should call you that either.