It is hard to explain to a loving person who can only give, what the refusal to receive does to would-be givers. If our gifts come out of the substance of who we are, to refuse our gifts is a rejection of our very self. At the same time, the turning away of a gift destroys the reciprocity of love. In place of mutuality, it sets up a hierarchy of love that makes the one who always receives and whose gifts are refused feel empty, powerless, and incompetent to love well, and so unable in turn to receive from the beloved with a grateful heart.
If the heart wanders or is distracted, bring it back to the point quite gently -- and even if you do nothing during the whole of your prayer but bring your heart back, though it went away every time you brought it back, your time would be very well employed.