It happens slowly as you practice daily. You will fail and start again. You will feel frustrated at yourself and your lack of discipline. Then one day you wake up and realize that it is just easier; it has become a default. Somehow, miraculously, the thought process evolves to more positive, happier and silver-lined with compassion and love.
Moments when you fall back into the old patterns become further apart, dissipate rapidly and do less harm to the human beings in your midst.
Gratitude = Happiness