Sorry you're having a bad week.
You can pick up your good habits again!
Some suggestions (if I may):
1. Prioritize. What schoolwork really has to get done now, and what can be done later? Can you use your time more efficiently?
2. Don't worry about being perfect. Unless you've got scholarships or something riding on your grades, do what you need to do to get a safe pass and learn what you need to learn. Don't knock yourself out for every last point unless it's important. (learned this the hard way)
3. Make one good decision. Even if you're still engaging in unhealthy eating behaviors, just do one little thing differently - drink water, get the low-fat option, get the smaller-sized bag of junk food. You'll be taking back control of your eating, and you'll have something to feel good about. Life's a process and a journey. You don't (you can't!) go back every time you screw up. You just have to keep going forward.
4. Write down your good decision, and then write some ways that you can do better in the future. And be realistic, don't just beat yourself up for slipping. Make this a habit.
5. Plan. Try to give yourself a variety of healthy, tasty food options that you can rely on without expending a lot of time or energy when you have other things to do.
5. Keep trying!
You can do it!