This is a problem that is widespread among naturally introverted students, or students who feel daunted about the prospects of having their opinions out in the open. Having been in your position, I perfectly understand where you're coming from and want to reassure you that it is only natural and there's nothing at all wrong, everyone has different learning approaches and preferences. Luckily, there is a great deal you can do and they are all linked by this one element: A positive working relationship with your teacher.
Everything revolves around you having a positive relationship with your teacher. Some students go through the entire year without having a single proper in-depth conversation with their teacher, and in English, this simply cannot happen if you have set ambitious goals for yourself. You have to understand your teacher and your teacher has to understand you and your writing and learning style.
Being hesitant to speak out in a classroom discussion does not stop you from going up to your teacher after they have delivered their lesson and asking them questions one-on-one. After most English lessons in Year 12, I would approach my teacher and ask her in-depth questions about the text, context and language analysis. I would ask her advice about my writing and ask her what she thought personally on certain viewpoints about themes and characters. Every lesson, I would get my own 5-10 minutes of 'personal time' with the teacher while everyone else in the class sat back and talked, or completed their work. Although it is good to use tutoring services and outside help, your teacher and the relationship you have with them is your most important point of contact in Year 12. Take the initiative and it will pay off immensely.
I've had students who have had such good relationships with their teachers and seen it greatly benefit their learning. But not only that, a past student who was in constant contact with their teacher through text and email after school hours (marking essays...etc) gained such a repore with his teacher that his essay submissions would 'skip the queue' of other essays she received for marking. During the busy exam period, his essays would get marked first. Such was their relationship and his show of effort in the eyes of his teacher that all his work would get priority marking even if it was submitted later than his peers. Now, I'm not suggesting that this would happen in your unique circumstances, I'm merely illustrating the point that a positive working relationship with your English teacher is the front to helpful discussion and is the strongest point of learning and contact you can possibly have in school.
So having troubles speaking/contributing in class? Email your teacher about the discussion. Talk to your teacher about what's good and what's bad with your body paragraphs. Ask your teacher questions about their personal thoughts on the character. The list goes on. Be prepared and you will learn everything you set out to learn and more.