From Chapter 7 ("The Mindfulness Path") of Toni Bernhard's How to Wake Up:
Moment-to-moment experience does not arise according to our desires. Sometimes the moment is pleasant; sometimes it's unpleasant. Cultivating mindfulness trains us to be calmly present for life's ever-changing joys and sorrows. This calmness of mind frees us from the suffering that arises when we cling to joy and resist sorrow.
Because unpleasant experiences are an inevitable part of life, mindfulness is not synonymous with joy: if you have a headache or have just fought with your partner or your children, careful attention to the present moment is not a joyful, pleasant experience. But mindfulness can be synonymous with waking up: this is how things are. Just this moment. Just this headache. Just this heartache. With practice, we can learn to respond to our experience with unconditional openness, even if that experience is painful—even if that experience breaks our hearts.