Don't worry. I know the life you have is rough and that you don't know if you are good enough for the world anymore, and that everything seems to be falling down around you. I know people must say this a lot, but trust me: there is always good in this world. Being depressed and sad won't fix anything. What would your Grandmother want you to do? Would she want you to feel heartbroken and in despair, or would your family want you to keep your hopes up? I'll give you some advice: help others. Listening to others' problems and helping your friends and family will make you feel better, it always helps me when I feel depressed.
So, I know you might not be Christian, but I want to let you know this: God created us to have a purpose in this world. You are not useless, you are one of the greatest people on earth. Feeling down won't fix anything, and bottling up all your emotions and pain won't fix anything, either. Just be you, be the way you are and remember that you are unique and not a mistake. We are all in this world for a reason, and we all should live a great life. I am terribly sorry you feel this way, and I really hope you have a better day. Know that everything will be all right. Don't focus on just your problems all the time, there is always a light in this seemingly dark, depressing world filled with nothing but hate. Focus on the good, and stop acting as if everything is alright. Trust me, everything will be okay in the end!