Think of motivation as the myth it is.... yeah there may be days where you feel like getting shit done, but they are very rare exceptions. You just never will feel like doing it, so you might as well stop sabotaging yourself and grind through things.
Checklists can give you some positive reinforcement as you get things done
There are also gamifying apps like Habitica. I didn't last long in them but they might work for you.
Find ways to make your work fun, or to implement some form of positive reinforcement. In my case I try to insert things that make me laugh into my work, but I have an awesome line of work that lets me do this.
Remove distractors some way or another. Make them hard to reach. If motivation is lacking, then sometimes this is all you can do.
Discipline is basically the grit to do shit when you don't feel like it, and it's something you build, like a habit. Disciplined people aren't motivated, they just are accustomed to pushing themselves through shit they don't want to do on a regular basis.
That being said, if you don't enjoy your job, will you be okay doing it for the rest of your life after school is done? 8 hours a day is a long time to pour on something you don't enjoy.