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.
Every time you forgive, you dissolve a condition you have placed on your own capacity to love. Every time you forgive, love is awakened in you more deeply and your capacity to extend love is increased. That is the nature of the journey.