Concentration is a cornerstone of mindfulness practice. Your mindfulness will only be as robust as the capacity of your mind to be calm and stable. Without calmness, the mirror of mindfulness will have an agitated and choppy surface and will not be able to reflect things with any accuracy.
Jon Kabat-ZinnSee for yourself whether letting go when a part of you really wants to hold on doesn't bring a deeper satisfaction than clinging.
Jon Kabat-ZinnIn any given moment we are either practicing mindfulness, or defacto, we are practicing mindlessness.
Jon Kabat-Zinn