hmm. i don't completely remember my year 12 experience, but i did have my fair share of burning outs during the year - yes i was quite often low on fuel

it's a matter of simply not overworking yourself - study as much as necessary, but remember that your ATAR does not define you, so you do not have to go killing yourself either - a social life is very very important because it kinda reminds you that you belong in a community and that you're not alone and that you have friends and family who support you
however, it is important to study constitently as well. i remember a quote: 'it's not what you do, but what you don't do, that makes you tired.' If you're productive consistently, you're up to date with work, you get it, you build confidence and therefore produce 'fuel.' So don't leave it until cramming, because it will kill you :/
my mistake was that i didn't study properly throughout the year. maybe i may have put in the hours, but i certainly wasn't productive as i could be. why? because I was lazy. because when i went to study german, what did i do? i read stuff. not actively even, but passive. think english language was the only subject i put work into to be honest :/
so in summary, burning out is very real and happens if you overwork yourself or if you cram in the last minute. so study consistently and don't kill yourself studying
