You can do anything if you set your mind to it. I myself am doing 12 hours a day right now, so I know what it entails.
Important things to keep in mind are to not wear yourself out too much. If you have been working the whole week, and one day you feel like taking an early night, do it, it is okay. Do not fight your instinct to sleep if you feel tired, because your brain won't register things efficiently then.
Staying motivated can be tricky sometimes. Write down your feelings, how you feel when you finish your target for the day, accomplish a big task. When you feel down, go back and read that. It might help in rejuvenating your energy.
Also, I noticed, that sometimes it is easy for me to do 12 hours a day for 2-3 days, but then I get tired after sometime, and start slacking. To avoid that, start easy. Baby steps. Start with 6 hours a day for a week, then gradually increase. It won't feel too much of a burden then.
Plan out your day in advance. This practice helps me a lot to stay motivated and finish my tasks early. Just before going to bed, I write down all the tasks I wish to finish the next day in my diary. Important thing is to maintain realistic expectations. Don't overschedule yourself, or you might not be able to complete everything.
Planning out a timetable might also help, though not necessary it works for everyone. Firstly write down all the time slots in your day, then start filling them with tasks, and adjust them as you go.
Plan out your priorities, what is urgent, important, both and neither. I am not saying don't use social media, or don't do fun activities. Relaxation is very necessary, but excess becomes procrastination. Try not to check your social media if you are going back to study after a 5-10 minute break. If possible, ask a parent or friend to just go through your messaging list, and notify you if anything requires immediate attention. You can answer the non-urgent messages during a longer break, because it is very easy to get caught up in our mobile screen ;). You can keep your weekends light to catch up with friends and media, and to do some fun activity which you like.
Also, try to plan your day such that you finish the big tasks early in the morning. It will keep you light for the rest of the day, giving you enough space to reschedule anything if necessary, and provides some, if not much, motivation.
You don't have to do everything by yourself too. If you are the kind of person who does well in group studies, plan a quiz session with your friend(s) or study group.
In the end, sometimes quality might win over quantity.
Also, you might want to check out the book "Eat That Frog" by Brian Tracy. You can find it in downloadable format here > https://oceanofpdf.com/authors/brian-tracy/pdf-epub-eat-that-frog-21-great-ways-to-stop-procrastinating-and-get-more-done-in-less-time-download/
It is a very nice and easy to read, which tells you how to just do it. I found it helpful over other self help books which tend to go deeper into the 'why' instead of the 'how'.
Hope it helps :)