I'm probably not the best person to answer this since procrastination and I are old pals but here are some suggestions

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1.
Watch motivational videos/ TED talks. (if you're gonna go on youtube, might as well make it productive). Same goes for motivation articles.
2.
Place some motivation in your study area. Put in some study scores you would like to achieve into an ATAR calculator and then print that off and stick it on your wall as motivation. Also, don't be afraid to lift your standards and goals. "Aim for the moon and if you miss, you will still be among the stars".
3.
Think about how awesome it would feel to go into a SAC prepared. and consequently get a good grade. I think once you get that one good mark, it acts as momentum and encourages you to foster that feeling. Conversely, think about the awful or indifferent feeling you get when you receive a lower grade. Was it really worth it, to play those games, especially when you're subconsciously feeling guilty about not studying?
4.
Set up a timetable. Grab a blank page, write out the days of the week at the top and separate the left side of the page into the different hours of the day, like the image below. Then, fill in your day with all the non-study related things, including breakfast, lunch, dinner, shower, you can even have like a couple session of free time/ gaming and then put in the work you want to do (make sure you break that down to simple tasks). For example, you may allocate a specific time for doing 3 exercise questions for Further and another duration for reading a paragraph of chem. Having a timetable usually helps my get started, though I have to admit, I do get a little too ambitious and jampack my day with things I want to do and then end up only doing like a quarter of it. So definitely avoid that. Start out by setting yourself small tasks each day and then build on from there

5.
Reward yourself. After you've say read 5 pages from your English book, have a small treat or play games but if you decide to do that or go on social media, I suggest you place a timer so you know when to get back to studying. Use this image as inspo hehe:

6.
Speak with your teachers, family and friends. It's good to gain an outside perspective and they'll probably give you more strategies and specific ways to improve. I would suggest sitting down with each of your subject teachers during a lunchtime or something like that and having a discussion about how you can improve. Trust me, they want to see you grow.
7.
Change your environment. Try going to a library or a cafe. Seeing other people doing their work is probably going to motivate you to do your own. Also, you kind of don't have an escape, as in you can't slouch on a couch or use your xbox so you're essentially forced to study. Changing your environment could also mean just taking a walk around your area to get some fresh air. Just try it once and see how it goes.
8.
Get excited about your studying! Be curious and find something to look forward to for each of your subjects. For example, imagine coming across a French person one day and being able to have a conversation that flows with them! Or, get excited about understanding how your body functions or what gives the world around you the properties it has through studying Bio and Chem. For example, the other day, I learnt how glowsticks work whilst reading my Chem textbook! It can be simple things that just encourage you to want to learn more.
Sorry for the long post and if you've heard a lot of this before, hope it still helps! Hey, at least you're seeking ways of improving and it's good that you want to take that on before your VCE journey starts. Don't beat yourself up but instead use that as motivation to prove to yourself that you are capable of doing well. I would like to reiterate that I also find it really hard to get motivated so I'm far from perfect and I really do need to take my own advice but I know that we just have to stick the process out and be persistent
