You hang on by taking each moment as it comes.
If you get really desperate then you need to go to the hospital.
I know it hurts. That's life. If nothing else, It's life. It's real, and sometimes it f*cking hurts, but it's sort of all we have. you arent selfish. the world isnt fair. I know how you are feeling and i know that words are hardly comforting but just know that you arent alone. And life is f*cking hard....i see your the same age as me...and us being only 17 we dont have a lot of power to change what is happening in our lives. But hang in there. Things seem bad, but they always get better at some point. I could be completely wrong, but i think you once said you were into photography...this might seem lame but have you ever tried taking pictures expressing how you feel? One time when I was feeling really awful and suicidal I walked down the road with my camera and took a picture of flowers stuck in the mud and things like that. and i dont know, it kind of expressed how i felt and then after i took a bunch of pictures i kinda felt better.
But hang in there...good times are bound to come.