I'm feeling your pain and I just keep procrastinating and not just sitting down and writing through it.
But that is the key, just sit and write. Don't try and make it the best writing you have ever done, just write and as you do, ideas will flow, the characters will literally tell you where they need to go and then you find you're back in the flow and off you go.
Also, write for a set time and then force yourself to stop. Walk away and let the mind mull it over. It will refine what you have done and then you can come back and fix and write on.
I sometimes find that I get blocked when what I am writing is BORING! So, I have to think of something to chuck in the make it more interesting. If I'm not interested when I write it, no one will be interested to read it.
Hope that helps (and hope I can find the time to write through my own block!!! ARGH!)