Breaking through that initial barrier, the first few words... that's the toughest part. The trick is not being afraid. You gotta walk right up, look it in the eye. Size it up.
Look for weaknesses, chinks, footholds. And then go. Plant that first word, even if it's not the perfect one - give yourself permission to toss it out later, but use it now as a starting step to get you up and over that monstrous wall of your own making.... formed from your insecurities and doubts and distractions.
After that, the words come easier, one by one at first out of no where, and then in larger groups, at times tumbling over each other in their hurry to get on the page...
It's similar to a dance, where the first couple on the dance floor isn't necessarily the best, just the bravest, paving the way for the others to step out of the darkness and join them, moving to the music of the story, until finally the dance floor - the page - is full.
And then you start all over again the next night...