I can identify with your situation. It sounds a lot like my first 20 years here on Earth. Honestly, I have found that the single best thing for warding off depression is regular exercise. If you could find a physical activity that you could make a hobby out of I bet it would be a step in a positive direction. Sports seems like the obvious first choice, but that requires organizing a lot of people, which isn't always possible. If you can't do that, you might try dance or martial arts. When I was about 18 I found a dodgeball league down at my local YMCA. I got plenty of exercise, it ate up a huge amount of time, and I didn't have to spend it alone.
If your back is too hurt to do any of that, Yoga might be an option. It includes a lot of stretching that might help it. In fact there are yoga programs specifically for low back pain.
When I was a stressed teenager I took up knife throwing for awhile. It was physical and a great way to release my anger in a controlled environment. A beginners set was like 10 USD at a nearby martial arts supply shop. They usually have those crazy metal playing cards which are pretty fun. Which reminds me, have you ever seen those people that can toss a real playing card right into a melon? I never got that good at it, but hell if it wasn't fun to try. Packs of playing cards are dirt cheap.
I realize these are all weird hobbies, but hell... I was a weird kid. Hope something here helps, and I hope you feel better.