Based on what I have read so far, you seem to be stuck in the classic trap of "trying to do everything at the same time or NOTHING". Unfortunately that is a strategy that has a very slim chance of success due to what you wrote earlier as well: willpower is a finite resource.
After you read this, get off the computer and go for a 15 min walk without stopping. It can be pacing around in your room, house, outdoors if weather permits. That will form your new baseline. Then commit to record EVERYTHING you eat for a week (or until next sunday) - no change in diet, just record what you eat. There is magic in there.
I suggest these 2 steps and see where this gets you!
PS. Willpower and Motivation is a difficult way to get better. It helps, but it is again, a finite resource. It is much much better to come up with a list of things (bite-sized) that needs to be done daily (or weekly) and just cross it off. No motivation required - just an acknowledgement that you HAVE to do it. Populate this list extremely sparsely to start, like 1-3 items. Examples: 1. Walk 15mins. 2. Sugar free until noon. 3. No sitting or lying down if you are not using the computer for school work.