I was 17 when I flunked out too
So even though you're technically a junior, you're still not really behind. Don't worry about your overall gpa. If you do well in all of your upper division physics classes and show a consistency in your efforts for a sustained period, your previous work will be disregarded after you explain yourself in your SOP.
Motivation is hard for some of us and you'll need to set both long and short term goals for yourself and focus on them daily. Pace yourself and make sure you have enough time and energy to not just get through a class, but to really absorb the material because you'll need to know it in years to come. You can do it, but you'll need at least two or three years of solid grades to show you've really changed.
One of the most important lessons for me to learn was that people care about grades not because they're supposed to show how smart you are, but because they show how mature you are.