What is repeated is learned.  If I had a penny for every time I thought or said this, I would be a billionaire.  NOONE, except YOU is responsible for how you feel.  I know this because as a young child I exercised managing my own thoughts because if I didn't,  I would have been left with the crap life dished me.  I wanted to be happy so I FOCUSED myself on the things that were good.  My friends, my teachers, my grades, my health, my ability to dream of a better life -  were the things that I held onto.  IT WAS A CHOICE.  I repeated it every day.  Some days 100 times.  If I had thought, "My life sucks.  I am hungry.  I am abused, neglected, unloved" - where would I be?  Those things were all true - BUT not in my mind.  I repeatedly told myself, "You deserve good things. You are good enough.  You are smart. You are pretty.  Look on the bright side."  AND that is what I got.  What is repeated is learned.  What you tell yourself is true.  Stop feeling sorry for yourself and change your sadness into happiness. 

Then people say "If it were just that easy, I would do it."  It IS that easy but you have to do it repeatedly.  The repetition is what makes it believable.  You must believe for it to be true.