Well, with all that's happened in your life lately it's not surprising that you're feeling unhappy. Losing two best friends is a terrible tragedy and you are allowed to be sad and cry and feel hurt, even angry over their deaths.
I've been through a severe depression but at the times when I WASN'T at my worst, I was able to enjoy things like going out with friends or spending time with my horse, etc... But as soon as it was over and I was home again, I would just go to my room and cry... feeling like there was no longer anything to live for. It's not all that uncommon. I know many people who have dealt with depression (my closest friends, my dad) and all have felt this way at times.
So please don't worry yourself to death about what it wrong with you... you have a right to grieve and maybe if you would share what you're feeling with someone (a parent or a friend maybe) you'll feel a whole lot better.
Have you ever considered grief counseling? Just an idea.
Take care of yourself, okay? You shouldn't have to go through this all alone.