i don't know why i am incapable of been happy for any long period of time.

i don't know why i am incapable of been happy for any long period of time. I have struggled with depression,, anxiety and panic attacks for years now. My depression seem to have improved a lot with medication and therapy, but most times, i am more unhappy for longer periods than i am happy. No matter what i do, no matter the situation, i am not a happy person.What is wrong with me and why  cant i just be a happy normal person. My mental state confuses me.