I don't know if it's necessarily low self esteem, it's the idea that NFs can get really really down without support for their ideals. Not only do you feel like you have a stupid idea, but then then if you're a particularly introspective NF, it starts into the huge downward spiral of starting to dissect everything and you start feeling guilty and meaningless and.. lemme just see if I can't find the quote I'm paraphrasing..
From "Portrait of an INFJ"
This is from an INFJ profile, obviously, but I'd imagine any idealist could fall into a similar trap....have a tendency to become severely depressed when their ideals go unfulfilled. It is amazing how quickly the INFJ's rich inner imagination can turn to discouragement when others don't readily support their cause. That which was inspiration now spirals ever inward toward self-punishment and deep-seated feelings of failure. Guilt becomes overriding and depression abounds. In such cases the INFJ tends to distort reality and to bury his or herself in a barrage of despair ending in, "No one gives a damn. How foolish of me to have thought otherwise."
EDIT: expanding on what Pink said. XD