I resat my A-Levels independently going from BBD to A*AB but that was only Year 13 so it’s a completely different situation.
Based off what you’ve said, just take the chance to resit Year 12 again. I know it’s frustrating but if you knuckle down and work hard there, you’ll have the basis to go into Year 13 next year with a great outlook and hopefully some decent unis.
In all honesty, getting CDU up to A*A*A or whatever in the space of a year is very unlikely, while universities would be able to see the amount of times you’ve retaken Year 12 anyway.
Go to a new college, possibly change the U subject and have a fresh start with Year 12.
What’s your eventual goal and where do you want to go?