The only solution is to start writing, without worrying too much in your first draft about style and length. Just get your facts and ideas down on paper, in a logical order as indicated in your outline, and you may be surprised how often problems clarify themselves in the process of composition. If the introductory sections cause trouble, skip them for the moment and plunge right into the substance of the thesis. It goes without saying that you are expected to write grammatically and spell correctly, but these are only the first things you should strive for. The cardinal virtues of good historical writing, to paraphrase Samuel Eliot Morison, one of our most readable historians, are clarity, vigor, objectivity, and grace.
Confused writing is the obvious result of confused thinking.
You must decide exactly what you mean to say before you can write it so that the average reader will understand. Use direct rather than indirect statements, the active rather than the passive voice, and make every sentence or paragraph an organic whole. And constantly, as you write, guard against special pleading. It is a good idea to read aloud sections that you are doubtful about, because the ear often catches errors that the eye doesn't see. It is very difficult to write with a graceful prose style, but very frequently the best writers are those familiar with the broad range of English and American literature.