I'm in year 12 so I completely get where you're coming from. Good news is, that you've realised it now and not like a month before the actual exams like I did. I'm assuming, based on your predicted grades of 8's, you're currently doing higher and are quite intellegent like myself. As such you should have plenty of time to study for your GCSEs. I suggest prioritising. Not completely though. Just find one subject that is not only your lowest current grade but also the subject you find hardest. Place this subject at the top. For me that was maths. Like you I was predicted a Grade 8/9. And let me also tell you that at the beginning of year 10/11 (I can't remember which), My report said that I was on a Grade 2. In fact about this time last year, I was on a Grade 6. Luckily I was already on the UniTracks programme from year 10, and they invited my to an intense 3 day, 4 night bootcamp for easter to work on a subject of my choice. Naturally I choose maths. But that wasn't until Easter. I didn't want to ask my parents for a tutor (for money purposes) so I took it to my maths teacher. She was hard to deal with as aperson but a very good teacher. I asked for worksheets on things I didn't get and asked her after school to re-explain things I didn't understand in class. And, as I'm sure you've heard milllions of times, my true saviour were past papers. That means applying the knowledge and than targeting the types of questions that you frequently got wrong. Literally two weeks before the actual maths exams I learnt about three or four topics that I had gotten wrong in three past papers. And I don't mean re-learnt. I mean I had no clue as to what this was. It was completely new. And thankfully I did, because it came up. So if you can do some kind of bootcamp, or worksheets. And past papers and then use your teachers to learn from the papers. Try not to panic. I know it's difficult but plan some kind of event as stress relief in advance about half way through your exams of a huge celebration after the exams. That's how I survived. I had planned to go to (my first) comic-con about half way through my exams. So when I started dying from the stress and lack of sleep I just told myself "just a little bit more. A bit more and then you can have a break. Do it for comic-con."
That being said don't ditch all of the other subjects too. Just try to get them to a suitable level. A level that you find acceptable. For me that was a Grade B, or at least a C. For those don't force yourself to go overboard. Just try to maintain that level. If you can find extra time for them, then great. I'd suggest splitting up your study times. Say you had planned to do 16 hours a week of studying (excluding breaks). And you have 5 subjects. For maths (the prioritised subject) do 4 hours. And then you have 12 hours left, that 3 hours for each of the other four subjects. But do add in breaks too. I try to stick to the 20 mins work, 10 mins play. If you go over, don't beat yourself up over it. Somestimes I'll do 25 mins work and 5 mins play if I'm on a roll, but do get at least a 3 min break for every 20-30 min work. And once every two weeks, of even once a week, schedule a day off to just relax and unwind. Because I promise it will get really nasty if that stress builds up too much, like mine did. Anyways. Focus on the hardest, level off the others and take breaks and have rewards. Don't overwork yourself. Sleep as much as you possibly can (6-7 hours minimum), eat, and vent to your friends and classmates. (They're in the same boat as you, even if that boat is the Titanic) also sorry this is soo long.