What the fuck is wrong with me?

I don't understand myself anymore. Why am I always so sad? It makes absolutely no sense. I should be happy. Those words are hammering in my head. Those four words...

You. Should. Be. Happy.

I have no reason to be sad. Life is fine. People are good to me. But still... I just thought I was over it. Yesterday, I smiled. It was nice. I had a nice day. And now... things that should make me happy, don't. I read something and it makes me so unspeakably sad, although it's not. It's other people's lives that make me sad. I feel like ever other life is better than mine.

How long has this been going on... how long...

How can I be happy again?