My girlfriend thinks I'm too good for her...?

For a while now shes been saying that im too good for her. Im that kind of guy who never gets mad at her and is always there for her to talk to or hug when she needs it :). I just found out that after we say goodnight over text every day she stays up crying For about an hour because "she isn't good enough" we had a sort of one sided fight just now where she yelled at me to "do her a ******* favor and be a jerk for once" which is something I really can't bring myself to do.. Every time I say she's beautiful or smart or nice she just feals worse let alone when I say I'm lucky to have her. :( I don't know what to do! I really like her... Gah! If you know what I can do please help me!