Keep In Mind What Writing Is
At the very bottom, writing is simply thinking onto paper in an attempt to show others what you think.
The Best Writing Is Structured
However, the best writing is structured so that others are drawn into your explanation in steps. It is focused thinking.
Reveals the Challenge of Writing
Thinking about everything is impossible and yet that is what they tell you to do as they tell you to consider the subject from every angle and consider all data. Unfortunately, that type of thinking can never be written down.
What You've Said May Indicate That You Feel Overwhelmed
It seems that you feel overwhelmed by the outside direction that forces you to think in a certain way. You are thinking about the thinking more than you are thinking about the subject itself.
Wasting What You Are Writing
We often feel like we are wasting our writing by simply writing things out which do not directly relate to the thesis, but sometimes it's a good idea to free write (free think onto paper) to learn where you really go with the subject.
How can I know what I think until I see what I say. ~E. M. Forster
A Possible Way In
Forget about structure of the thesis and all of that. Instead, for a little while, concentrate entirely on your subject and ask some questions like:
What exactly is it that I am writing about?
What conclusion am I driving toward?
Write A Mini-Dissertation
Now take that information and write an extremely short version of your dissertation as fast as you can. Call it Draft Zero.
Simply write one line for each paragraph which should be in your dissertation.
Write the introduction, body, summary and conclusion.
Keep in mind the reason they do not exactly describe what should be in each part of the dissertation is because it is expected to be different from different authors.
There Are Books To Help Too
There's even a "Dummies" book
Writing A Dissertation for Dummies - amazon link