I'm This was part of my problem, but my 3 years out of school I worked manual labor and those jobs kicked my ass. I knew I didn't want to do that ever again, so that helped my focus a bit when I went back to school.
But it was still tough. I didn't really want to study. I started small, before I took hard classes. First, I resolved to never, ever miss lecture, and to take notes during lecture.
Then I made myself read the book and my notes for 15 minutes a day, in each subject. It was short enough to not really feel like work.
The third thing I promised was that I would ALWAYS do the homework. All of it. It was hard at first, I didn't realize math and intro chemistry could take such a long time. I pulled 5-6 all nighters my fist quarter back at school. I still would procrastinate but I refused to let myself miss easy points. I also didn't cheat, I actually did it.
I didn't know how to study, so the day before exams I just looked at notes and did some problems. I passed my first quarter of precalc, human anatomy and intro chemistry with As (I wanted to be a nurse, initially)
Next quarter I took calculus 1, gen Chem 1 and gen bio 1. I did all the same stuff, but I was slowly getting better at it, and I would accidentally work for longer than I set time aside for it. I started studying a few days in advance for exams instead of the night before. But calc 1 and gen Chem 1 was where I really developed my work ethic.
We were assigned maybe 20 problems a week in calculus and 30-40 problems a week in gen Chem. I did them all. I pulled 1-2 all nighters to do it at first and felt like such shit after that I started doing them the day ornday after it was assigned so I had the whole week.
It was really a slow progression. I can handle large work loads now (I can do 50 Ochem problems + reading in 2-3 days, same with math)
Perhaps most important: I have NO social media. It cost me a D on a math final once, and I deleted all of it. I read books now.