the easiest way for us to gain happiness is to learn how to want the things we already have
There is a difference between contemplating something bad happening and worrying about it. Contemplation is an intellectual exercise. Conduct such exercises without affecting your emotions.
Do not over-love the things you enjoy. Be the user, but not the slave, of the gifts of fortune.
If you periodically embrace discomfort, you're more likely to be comfortable than someone who tries to avoid all discomfort. You'll have a much wider comfort zone than others and will therefore feel comfortable under circumstances that would cause others distress.
Retrospective negative visualization: imagine never having had something that you have lost. By engaging in retrospective negative visualization, you can replace your feelings of regret at having lost something with feelings of thanks for once having had it.