What is it you fear most as you sit in front of a blank screen? Perhaps the fear of rejection holds you back from putting words on paper. You know your work is likely to get rejected by publishers and agents because the experience of others told you to expect it.
Maybe you fear humiliation. Putting yourself out there on paper opens you up to all kinds of criticism and ridicule. It's really hard to be vulnerable in your writing because the critics' sting hurts that much more.