Fear and poor self-image and writing are a frightful ménage à trois. “Master your fear” is a pithy phrase for this kind of fear. Self-image issues do not need mastery--they need elimination. There is no bargaining with a poor self-image. Like a terrorist, it only deserves ire and elimination. Stomp on it first, and then address the causes.
The causes of poor self-esteem are legion, as are the effects of fear on writing. Nevertheless, with a little bit of knowledge comes a lot of understanding. Take the query process, for example.
Fear of rejection is an honest fear having biological roots. We are genetically pre-disposed to fear rejection because rejection, not too long ago, meant death. Literally. On the evolutionary scale, rejection was starvation. Hyperthermia. Hypothermia. Being eaten.
But ah, the query process! How can one fear agent forty sending out a rejection, when one was rejected by the prior thirty-nine? After a while, it becomes an intellectual challenge and a test of tenacity (hopefully not stubbornness with a side of whoops).
Repetition is a great fear killer. Writing is a creative endeavor. Eliminating a poor image of self is difficult as the writer rallies against subversive forces both within and without, but creativity flourishes in the absence of fear. Even writing about fear is easier, as once can see it for what it is.
Thus one of the answers to fear, fear of rejection, fear of snide comments, fear of hurt feelings, fear of not getting there, is perspective.
A ferociously yucky animal did not eat you today. And that makes today wonderful!