why do i always feel down

I'm not sure why I feel down and when people ask I don't know what to say, I feel so worthless and like I get in people's way, and everytime I get worked up I just feel like I'm over reacting and no one cares, I'm not sure who to talk to or just to hold it in so I don't bother anyone because I always feel like I'm creating problems for no reason and I do t know how to stop it