I'm in the same situation as you.
I used to muck up in class, not pay attention and pretty much avoid the tasks given to me. I also gained some mental issues during the start which is why I started skipping school. Though, I really don't want sympathy. So, I'm also trying to get back on track, I'm re-doing my HSC year and setting my goals. I want to work towards them, I know with effort they can be attainable, the goals aren't drastically unrealistic. But, don't sell yourself short of what you can achieve.
I'd highly recommend getting in touch with your teachers; express how you want to do well and ask what can be done to do that. They'll surely help you out, I remember saying that to one of my teachers and he decided to give me extra lessons at lunch along with other students who wanted to put the extra effort. It's not a thing to be ashamed of; it's what teachers get paid to do.
Procrastination. Now, try to avoid that and put initiative into being organised with assignments/homework or notes. Revise the notes and when given assignment tasks read the criteria carefully. I never paid much attention to it, till one glorious day. I read what was needed and put in just that. I received 19/20 for that task, I forgot my graphs and ended up drawing it because I forgot : (.
I actually find visiting this site, highly motivating. I really like to see the effort people put as it makes me aspire to do the same.I feel I'd go on too much with the topic, so basically try your best, study hard but live a little (( don't burn out )) and aim for the goals that you want to achieve.
Best of luck!