Mindfulness is a serious matter, no doubt. There is also a real danger in taking one's practice too seriously. A persistent frown, jaws clenching, shoulders becoming stiff, shallow breath, knot in the stomach . . . these may all be manifestations of an overzealous attitude, a case of practice taking a wrong turn. U Tejaniya spends a lot of time on this, teaching about the right attitude:
You have to double check to see what attitude you are meditating with. A light and free mind enables you to meditate well. Do you have the right attitude?
There is a game I play often during practice, and that helps me stay light and concentrated. I call it playing catch with the breath. Sitting with the intention of resting in the breath, I allow myself to be surprised by each new inhalation. Over and over, breath out, noticing pause, and then, the delight of breath coming in. Sweetness of body being breathed once more. Being grateful for yet another moment, alive.