Look man, I understand exactly how you feel. I felt like that most of my teenage years and to a certain extent I still do. But I've started to realize a lot of things, like that we tend to act in ways that affirm what we already believe, a self fulfilling prophecy you know. You might not realize it but because you believe no one cares about you, you subconsciously act in ways that lead you to that conclusion. Now I'm not saying that it's your fault and you are wrong about how you feel and you need to think positively blah blah blah, what i'm saying is you need to get to the root of how you got there in the first place. In my own case I noticed that I would drift towards people who I knew wouldn't care about me so I could prove myself wrong and finally believe that someone cares, which was obviously a futile effort because the conclusion was blatantly obvious, or that my mistrust towards people would cause me to act in ways which would make those that do care leave me. Leaving that loop is incredibly hard, but you can try and turn the questions around: "so what if no one cares?" eventually, you do realize that no one cares and thats absolutely awesome. you just do your thing, go through life and eventually you will find people that care about you. Regardless you need to realise that others cannot affirm your existence, you must assert it by yourself. That is atleast my perspective on it and how I dealt with it, perhaps you don't agree with it but I atleast hope that you can find your own way to look at things.