Why do I feel so sad

So I'm bitter, angry, confused, and it's not becoming of me really. I've tried to let it go, but it creeps up when I should be the happiest.i can't understand why I'm so upset all the time. When life is good, I get uneasy, when life is bad I get depressed, when I'm trying to help others, I get jealous over their happiness, so what's wrong with me?