Its good that you can show them the history of how you went about writing the essay - shows that you did not use an essay mill. For a second year to only rely on 3 academic sources all from the core text is a little weak (but nothing to do with plagiarism). I am sure you were taught how to reference properly in year 1 so why did you not follow that?
You can only plead ignorance - it will be up to the uni to prove that you had been taught how to reference correctly. Worst case, you will get a zero and be asked to resit the module. Depending on the regs, your resit mark could be capped at 40% which will impact your average grade for year 2. If your year 2 marks go towards your final mark - in lots of unis that is the case - then you have to do well in year 3 to get your marks up.