0 the words won't flow

I am not sure why it is that when I want to actually put things down into words it is so hard.  My mind rambles on and on with insane thoughts and ideas all the time and yet when I actually want to lay them out they escape me.  I wish I understood this need of mine to write.  I also wish I understood my need for perfection.  I think that is why I find it so hard to write.