Reach to Teach

How do you teach someone to learn to choose gratitude and happiness? How do you remove a fixation on negativity from someone who is consumed by their own darkness?
I like to think i taught myself, by seeking out those more wise than i, and listening to them. They taught me the proper vibration was what is most important, that your attitude determines your atmosphere, and to choose thoughts practically that support love, kindness, compassion and joy.
How do you introduce someone into making that choice instead of allowing regret and lamenting to put the perceived darkness of their past into their future? How do you teach mental health and wisdom?