Why can't I feel anything but pure sadness?

I’ve been like this for so long and I don’t know. All I feel is sadness, pain and anxiety. Whenever something good comes up, I’ll feel okay for a moment, then I’ll just go back to feeling down again. No matter what I do, I just go back to where I started. I feel nothing. I feel slow, empty, tired and not human at all. No one is there for me. What should I do?