sorry to hear about your challenges last year, my daughter had a really tough yr12 and has struggled with motivation in yr13. If it helps this is really common, and figuring out what will work for you is a really useful skill that will help you through life
So many possible answers to your question
With each subject - do you know what you need to do to close the gap? A 'C' to an 'A' looks like a big gap but if you know what you need to work on then that's a good start. If you can make it tangible and have a plan of attack that's a start. If you dont know yet you need to sort that
With your overall goal - do you know what you want and why? Again make it tangible, be as clear as you can why you are doing this and keep reminding yourself when you struggle with motivation. My eldest put a photo of the 2 unis she really wanted to go to on the wall by her desk as a reminder of her goal. It could be a job you want or almost anything that would give you a clear goal / target. If you dont have a tangible, desirable goal it can be hard to get motivated.
What is stopping you getting started? Or do you start and quickly get distracted? Understanding what is getting in the way will help. One approach to motivation is called (I think) "swallow the frog", get the worse bit out of the way first. Another approach is pomodoro, break things in to chunks and nibble away. You can research other approaches and see what might work for you
Often lack of motivation comes from a lack of clarity of the task ahead, it feels too big or overwhelming or pointless. If you can chunk it down, ignore the big picture and just do the next thing that must be done that can work too (you read Touching the Void by Joe Simpson?)
And don't forget to be kind to yourself, celebrate when something good happens. And ask for help if you need it, FWIW one of the things that improves my motivation more than anything else it talking to other people
Good luck