When applied to writing, the word "structure" can be confusing. Even writing which is very hard to understand structures information in some way, so that a marker's comment that an essay "lacks structure" can seem as pointless as the king's remark that Mozart's music had "too many notes".
What the comment that an essay "lacks structure" really means, is that it is not structured in a way that is orientated around the needs of a reader who doesn't have your degree of familiarity with the topic. Another way of making the same point is to comment that the essay "reads like a series of notes", since note-taking is a form of writing which legitimately adopts a writer-orientated perspective.
The essential problem of structure you face as a writer is how to re-orientate the material in your notes so that it becomes relevant to your reader's needs. The introduction to the essay plays a very important role in orientating the material to your reader, because it is here that you do most of the work of framing the topic, but later parts of the essay need to maintain the motivation of the material by means of signposting.
It is worth remembering that the spatial metaphor implicit in the word "structure" is slightly misleading. The reader experiences your writing as something that unfolds in time, like a piece of music, and this has important implications for understanding your reader's needs.