Hey,
First of all - I just want to say, you're not an idiot, you're not stupid, you're not a failure or any other degenerative features you may think you are. The HSC and your father's expectations are just an obstacle that you can overcome. At the end of the day, the mark you'll get is what you'll get. There are definitely, 100%, ways of improving your marks for your externals and getting a higher ATAR than what you're expecting. Being ranked last in your school does not always, definately, 100% mean that you'll get a ATAR that's bordering a mystery mark. If you try hard for the externals, you should be able to get a higher mark. Have you tried looking into scholarship or bonus points/ other features that can potentially raise your ATAR? I feel like the stress is hindering on your academic progress, and is causing you to make silly mistakes. Remember, as cheesy as it sounds, if you truely believe you can do well, you will do well. That top maths kid doesn't believe that he/she is going to get 0 in their test. They're confident and they know that they'll get a good mark. This assurance comes from doing past papers and getting high marks in those past papers. Ideally this is where you want to be at.
Secondly - it's awesome that you have a tutor. There's nothing to be ashamed of if you don't get a question right after you've been taught it - give your brain time to process the answer. Everyone started off as a beginner. You just need to work the extra mile to catch up to your classmates - but I can promise you it's not hard. Chunk your workload first - so maybe do one exercise of maths today, review your notes for biology tomorrow etc. As you go through these excersises you'll start to figure out what you know and what you don't know. The more you do, the more you'll know and the more confident you'll be. Really start thinking you can do something and build up your confidence because it really, really, really does affect your performance. You have 5 more weeks of school (I think) and the HSC doesn't start until something like 3 weeks after school ends. You've got HEAPS of time. Don't be stressed or worried if you've failed to complete whatever workload you've assigned to yourself for the day - you still have a while to go. Studying is about being consistent, try not to cram if you can. That being said, while you should give yourself frequent breaks and ease yourself into studying, don't get complacent. Try and study at a place where you won't have that many distractions either - so e.g, if your father is stressing you out, maybe study at a library?
From your evaluation of your subjects, I would say that maybe you're struggling with memorising? Have you found an efficient way to retain information? For example, I like to read out loud and that helps me retain information. Try memory techniques etc if you feel like you're struggling. English - I know 800-1000 words of an essay seems like a lot to remember. Break it up, memorise paragraph by paragraph. Take 2 days to remember a paragraph, then on the third night write out the paragraph you remembered. Repeat with all your paragraphs - you can break your analysis up and use recollection techniques as well. Understand what you're trying to say. If you're really struggling, maybe consider findng a tutor? I know it's a bit late, but the externals does count for 50% of your mark.
Chemistry in the HSC course, unlike prelim, has alot of rote. So although you need to understand it, kinda, it's definately achievable. Practise is the key. If you're really struggling - just look at the textbook. Conquering chemistry I find is pretty good. Read it, and try to understand it. Then compile a list of equations and memorise that. If you understand the content, you should do OK. Then practise past papers. You can also make your own summarised notes to make sure you understand everything.
Lastly, you've identified exam pressure as the concern for the rest of your subjects. (Good job on identifying what's wrong! You're already half-way on the road to improvement! Now all you need to do is fix this mistake) Ask your tutor and/or teachers. Remember, in an exam, you're not aiming to answer everything if you can't. Don't stress if you don't get all the questions - you're aiming to maximise your marks. So if you don't know a question, skip it, leave it for later, and do all the ones you can first. Similarly in a question - if you don't know some components of it, don't stress. Answer what you can. If it's more than one mark, try and figure out what the other marks are for. Again you're not aiming for full marks, you're aiming to get all the marks you can. E.g, in chemistry - if it's a 4 mark question asking about nuclear isotopes. If you don't remember the equations for examples of isotopes, don't worry. It's probably only 1-2 marks of the 4 at most. Realistically, you can still get 2-3 of that four marks. etc
Hope that helped and all the best!