The way to be happy, sane, and mentally well is not to become unhappy, insane, or mentally unwell in the first place. But given that no one manages to do this, the next best thing is to look for things that work to bring you to greater happiness, sanity, and mental wellbeing, then do those things. The practices of Zen, including mindfulness, achieve these goals, but you have to actually do them, and do them often enough and rigorously enough for the cumulative effect of your efforts to result in what you want to feel - namely, happy, sane, and well. It is simply a matter of committing to the task.
Mindfulness is taught as a therapy for those suffering from stress, etc. But it was originally a route to a better way of living life, within the anti-philosophy of Zen. People are obsessed with plans and ambitions. But when goals are achieved the result is often the absence of any satisfaction. Zen teaches us to focus on the present, and enjoy the simple beauties of everyday life. It also has implications for how we can be as a society and as a species. I explore all of this here.