An adjective for a person who won't readily accept things?

I'm looking for a word in English that describes a person who wont easily accept what you tell him/her. For example, if you tell her you're beautiful she would say, "No I'm not." If you tell her she has changed she will say, "No, I haven't." If you tell her things will get better she will say, "No, they won't."

And I'm not looking for the word "depressed" – lol.