"I'm stressed" means that you are under pressure and worrying. "I am stressed out" means almost the same thing but has more of a nuance that you are at the limit of your sanity. "I stress out" is a transitive verb, so it is okay say "You stress me out." You can use stress as a verb and say "You are stressing me," but that is less common. "I'm dressed" is the opposite of naked. If someone is dressed, that means they were naked or only in underwear before and just put on clothes. "I'm dressed up" is similar but has an extra nuance of being dressed in pretty and fancy clothes.