Thinking in a positive way is great.
Trying to get rid of the negative thoughts is great.
But sometimes asserting positive sentences won’t work on you if you suffer from severe depression, anxiety or phobias.
It is OK to listen to sad songs, they can help you push out the sadness that is inside you. I used to sing that kind of songs and it was like expressing all my pain away. It is also OK to write about how you feel to heal your wounds. Or to draw the monsters that torture you if you have drawing skills. Or to moan in therapy.
However it is good to also try and spend time with people who are fine and will assert positive things about you. Usually, when your self-esteem is really, really low, it is easier to believe good friends’ positive words about you than your own positive assertions. It is faith more than the assertions that help you get in a positive mindset. When you have gained strength and have started feeling better, positive thinking and higher self-esteem will become natural to you.
So, to summarize, don’t blame yourself for not being able to be and think “positive”. Express yourself first.