Adding to everyone's opinions here, I know how you feel and I've been there before. I ran into a similar issue after realising that commerce just wasn't for me after Year 11 Economics. I agree with Joseph41: you'll still have strengths and weaknesses somewhere.
You've mentioned you enjoy(ed) the idea of studying Law and your subjects (By the way, pardon my ignorance, but what were your subjects?), so how about doing a broader degree? I know that sometimes simply immersing yourself in uni could help you figure out what you want to do with yourself. A broad degree will let you do that and probably allow you to do the odd elective that you enjoy here and there.
I'm not entirely sure how you do your preferences in NSW, but, if it's anything like Victoria, factor in some broader degrees in those preferences and transfer into Law at a later date, if that's what you decide to do at a later date. Leave some of the law degrees in your preferences so you still have a solid chance to study them. Alternatively, a double degree could also be helpful if you're too undecided but like two broad areas.
Can't even go into STEM subjects coz I'm it even good at them either. I genuinely have an interest to some degree in all subjects I did in year 12 and of course, law, but after LAT result I feel f***ed. -snip-
Assuming I have to go to uni next year and of course, I am interested in many things but just not good at them, what do I do?
P.S. You don't have to be 'good' at a particular area, whether that be STEM or not, to do a degree in the area. In all honesty, I'm not great at biology and I hadn't done biology since Year 9 until I started uni. Still went on to finish an Honours degree in the area because I liked it. Moreover, according to high school, my worst subjects were Chemistry and Japanese, but now I have minors in both areas, purely out of enjoyment. You should be doing a degree mostly for enjoyment and/or a stepping stone towards your intended career.