You don't become Kafir by breaking a promise...
You become kafir when you declare there is no Allah and Muhammad is not the Slave and Messenger of Allah... (ASTAGFURALLAH!)
This world is a trial. Allah tests us in this world. Sometimes we fail and sometimes we Pass our tests. But When we fail, we should not loose hope in the Mercy of Allah!
Allah is the most loving being in this whole Universe. Allah is Love, Allah is the Creator of Love !
Inshallah if you pray for forgiveness from Allah. Allah will surly forgive you! And Inshallah if you keep on the path of Allah and always try to make Allah happy, You will see your self that inshallah you will become close with Allah. You will become the one Allah loves...
Don't worry if you fail... But keep trying Keep trying !!
But of course there might be a kafara you have to give because of the sin you have done. But that depends on what type of Sin it is...
BUT YOU ARE NOT A KAFIR... YOU DO NOT LEAVE ISLAM BY BREAKING A PROMISE