There is no secret, you just do the next thing you need to do.

Getting started is the hardest part.

The more things you get done the more you realize that you feel way less stressed out after you check things off your list.

Make a list, get the most urgent things or the easiest things done first. Do things that can't be done again first. Doing something that you won't have to do again means you have one source of stress eliminated for good.

Save gaming and watching stuff until the end of the day. Only take a day off if you've accomplished certain goals.

Time spent being depressed is worthless time. You'll never look back at it with any fondness or value. Keep doing stuff while you are depressed. Do the hard stuff while you are depressed. The time is mentally/spiritually valueless so grind out unpleasant tasks, you aren't going to feel better or worse during these unpleasant times but when you come out of them you won't have even more and worse problems to deal with.

Stop making excuses. Go to class every day. It's the easiest way to raise your grade, whatever bullshit excuse you make to not go is just a bullshit excuse.

Better sleep and diet might be good for you but they aren't going to make you more productive. An iron will doesn't give a shit about weather it's well rested or fed, it just gets the next thing it must get done.