I used to experience this, a long time ago... A feeling like... everything was so bad, for so long, that if I allowed myself to be happy just one slight little bit, then someone would destroy me. It was kinda based in fact too, because if my primary abuser ever caught me being too happy, or taking pride in something, he would immediately destroy it and beat me into the ground. Same happened at school too. So I just learned to hide any happiness deep inside, or at the very most, express it when no human-scum were around.. (Lol, that was actually my term for the human race, at one point). They take so goddamn much from us.
Eventually though, I did learn to be happy, or at least have fun. They aren't exactly the same thing, but one leads to another. When I gave up on life as an absurd joke, an utter farce... That's when things started to get better, just a little at a time. I'm not suggesting anybody do this, precisely, but... there is a certain freedom in just declaring that "Life is shit, might as well have fun." There is no escape from life... but.. for just a moment, to have a drop of fun, every once in a while. It "trains" you on how to enjoy life. It's like... baby steps.. just a tiny exposure helps you to feel comfortable with happiness over the course of time. Yes, the sky will fall. Yes, the horror will return. Yes, all your problems will still be there.. Later.
In the meantime though.. you'll be a king.