Hi Super Sonic,
I think your parents' relationship has a major factor in your depression then. Because you sound like you take a large chunk of responsibility for your life, but external factors are affecting your state of mind and making it hard for you to become motivated.
My parents fought fiercely in the past and recently as well and I know it really, really, really, and I mean REALLY sucks. And from the sound of it, your case is even worse than mine so I can only imagine the horrible state their fights must put you in.
But I believe that if you want to help yourself, you must first accept your current situation and TOTAL responsibility for your situation no matter how terrible it is. And this is not speaking from a holier-than-thou attitude. I sincerely know you must have it 10000x worse than I do. But I want to help you help yourself.
Personal development is so highly effective and even more powerful than external motivation because you make your own motivation. Thus, we must begin with ourselves to solve our problems. If you manage to at the very least accept your current situation, then you will make prob. one of the most life-changing decisions in your life (although you prob. won't see that until many years later down the line).
Find something you love to do. For me it is exercise. I vent all my frustrations and stresses through exercise. When my mom used to get mad at me, it would push me to work even harder. In fact, the stuff I thought as I vented would prob. land me in jail for life LOL. But, the important key is to have a healthy medium to vent, ESPECIALLY if you're life is not stable and positive. I highly recommend some form of exercise.
You can really think about all the killing, punching, and mutilation you want to do to people who piss you off without ACTUALLY doing it (well I hope not!!). It's a good thing my mom doesn't know what I used to think about her when she made me mad
Here's another good quote, "Pain is weakness leaving the body." Bring it on!