Kurt could be insecure about his body, his voice, his looks, his weight Blaine's love for him, etc..
These could be set in the 'Born This Way Episode' the 'Sexy' episode, or Sebastian's existence in the series. I especially love ones where Kurt is concerned about his weight.
Pretty much whatever insecure!Kurt fanfictions you can find I'm alright with. These fics can be any pairing with Kurt, but I prefer Klaine.
Also, I would like some fics where Blaine finds out about Sebastian always insulting Kurt whenever he's not around?