"[...] the keys to happiness lie in mastering the mind, and not in changing external factors in our lives."

I have been interested in mindfulness for years, but I found it very unhelpful to follow strong statements like this one. In many cases I found pro-actively dealing with problems and trying to improve my situation is eventually easier than trying to keep my mind from them.

There is a balance between being mindful and content in the present on the one hand, and being analytical/critical/discontent on the other. Of course, most people in industrialized societies could certainly profit immensely from shifting more towards the mindful state.