About Happiness Or Chaos Or Childhood

My theatre-teacher says I always make sad characters. When I am able to be more joyous, I must put a bit of pulp in it. I.e. the lady that says the dad-to-be ... <>.
Why am I so tragic and sad when I try to act (even if it is improvization, it's a way of playing, I suppose)?
I feel myself a joyous people. A little melanchonic, especially in this period, but full of hopes.
What stops me? What stops my skill to be happy on the scene - or patinoire, as we improviser call the acting scene?
It cannot be only the need to sleep... I know I am tired, 300 km or a bit more by car in a day, a 5 hour lesson in Milan and so on are quite heavy... but there must be something more.
Why I ask you that?
Because you don't know me so much, but you have different way to think. I always search for various opinions.
When I was child, the question I made to everybody was "What's wrong about me?". Nobody answered, but everybody continue to avoid me.
Now I learned to make question less strong. But I continue to want to discover my faults, my mistakes, my lacks.
It's a period for questions.... excuse me...
In a certain way, I talked about chaos in my life, about my childhood and about happiness... but not in the exact way I intended to. I'll talk better about them in the next future.