In nearly every translation of Yoga Sutra II.42, Santosha is interpreted as the greatest happiness, the underlying joy that cannot be shaken by life's tough moments, by injustice, hardship, bad luck. Contentment is really about accepting life as it is, it's not about creating perfection. Life offers whatever it will to you, and you ultimately have little control. Be welcoming of what you get.
You can practice this on the mat quite easily, by acknowledging your tendency to strive to do a perfect pose and accepting the one you've got. The process of Santosha is relaxing into where you are in your pose right now and realizing that it is perfect. Santosha can be similar to the deep relaxation that is possible in Savasana (Corpse Pose). You can't run after contentment, it has to find you. All you can do is try to create the space for it.
If you release your mind from constantly wanting your situation to be different, you'll find more ease. It's not fatalism, however; it's not to say you can't change your reality. But just for the moment, can you let go of the war with reality? If you do, you'll be able to think more clearly and be more effective in making a difference.
During those times when you don't feel content, just act for one moment as if you were. You might kick-start a positive feedback loop, which can generate real contentment. It might feel absurd when your inner landscape isn't shiny and bright, but the simple physical act of turning up the corners of your mouth can have amazing effects. Smiling can change everything. Practicing smiling is like planting the seed of a mighty redwood. The body receives the smile, and contentment grows. Before you know it, you're smiling all the time. Whether you're practicing asana or living life, remember to find joy in the experience.