The reason you are plagiarising yourself and WHY it actually is plagiarism is because you have already been awarded a MARK for those words, in that order, on that line of argument. Those words were submitted and you've already taken credit for them.
It can seem unfair especially when some points and lines of argument can be similar from module to module but that's the point; that you think and write and research, you expand your knowledge, you should never HAVE to repeat and nor should you because you're going further into the subject and are showing that you possess wider reading skills, which carry hefty marks.
And yes what FatherReboolaConundrum mentioned. There is a huge logic to Turnitin, and it's very difficult to get past it for all the right reasons. Academics can tell very quickly (as can admin) whether the percentage is just or not, it takes expected terms into account (like the title). We had a student, who was gobsmacked when she found herself in deep trouble. She'd used an essay that she'd found online, because she hadn't studied her subject enough, she didn't realise the author of this essay was in fact a professor at our university.
The only way to impress, gain marks and get the mark you deserve is through a lot of reading.
Hand something in, whatever you do HAND SOMETHING IN! If you fail this module, it's not the end of the world, you can retake the assessment, you may only be awarded 40% maximum (a scrape pass) but it won't fuck up your studies entirely. Too late for an extension now I should think... I think you need to book a session with your personal academic tutor ASAP.
Don't knick yourself, it's the academic language and rules you need to nail, so seek help from study skills. I struggled with this side of things too, it doesn't mean you should give up or can't be very good. My grade massively improved when I really understood the game play/structure/rules. Good luck.