Go back to the first chapter or two and read what you wrote without noticing the typos and the occasional 'did I really write that nonsense!' moment. Then go back to where you are stuck and try again. Or put it away for a while and do something else quite different - then read it again. The first way helps to focus on what you were starting out to say - the second adds an objective element to your sight. Good luck with it!
If you're having trouble getting it down, maybe try to narrow down different parts and focus on them one at a time. I think that sometimes we get overwhelmed with all that we are trying to communicate and then it just doesn't come out the way we want it. So, breaking it down can help. I also agree with cosette, you can always edit later
When I need dialogue for my fiction I go to where I know there are people that talk the way I need them to talk, and I observe (lol eavesdrop) I'm lucky that I live in a big city and have access to many populated places such as malls, courtrooms, and even City Hall.
You say you know what you want to say but just don't know how to say it... well this might help.
Just start writing, stream-of-conciousness, and see what comes out of it. Or pick up a book from your shelf, open it, and start typing what it says, until your own words start taking over. Just be sure to go back and change the beginning when you're done.