I've had to make a lot of presentations and the difficulty you have is very familiar to me. Can I ask if you are using a presentation aid like **********? One of the good things about using aids like that is that it forces us to summarise what people are looking at, even though our own notes might be as comprehensive as ever. So the discipline of writing the presentation helps us to think in terms of those "accurate" summaries which don't dilute our overall message. Also, I find a timed run through of my presentation helps me to sift what needs to be said from what might be said in response to a particular question. I have used a book in the past which I just got down called "Agreed! How to make your management communictaion persuasive". It says the five objectives of communication like this are all to do with your audience - getting them to Hear, Understand, Agree, Take Action and Feedback. And in case you feel that that's a bit too "commercial", I work across all three sectors. Management tools are just tools.
Hope this helps.