I've signed up to do NaNo this year after mulling over my new book for several months. I'm at the point where I fall asleep dreaming about the book and about the characters and how I would use them. But now that the day to start is looming, and I know I need to write this stuff down before I forget it, I'm paralyzed and completely unable to get anything down on paper.
I know I'm my own worst enemy, but how do I get over myself?
I know that feeling all too well. The ideas are all there, and in your head they make so much sense, are so compelling and so beautiful. Yet getting it onto the page is nigh on impossible.
To be honest, I don't have a whole lot of advice about how to get past this other than to just write. Give yourself a time limit - 20 minutes maybe - and just sit down and spill as much onto the page as you can do in the time. You may only get a couple of words out or you may unleash a torrent. Whatever you get, embrace it. Use it.
Then do it again.
My other suggestion - and it probably isn't the best one I've ever given, but it works for me sometimes - is to go into NaNo without anything written down. Sit down on Sunday and start writing and see where the story and the characters take you. It sounds like you have a really great handle on the story you want to tell, and how the characters engage with it. Maybe you're overthinking things and putting pressure on yourself unnecessarily.
Whatever you decide, enjoy it. And please don't punish yourself if things don't go as smoothly as you hope. Writing isn't always easy, and NaNo isn't for everyone. No one is going to die if you don't hit that magic 50K, so there's no need to kill yourself to get there. Or risk your mental health.
Hope this helps...