Letting go is not such a foreign experience. We do it every night when we go to sleep. We lie down on a padded surface, with the lights out, in a quiet place, and we let go of our mind and body. If you can't let go, you can't go to sleep.
Most of us have experienced times when the mind would just not shut down when we got into bed. This is one of the first signs of elevated stress. At these times we may be unable to free ourselves from certain thoughts because our involvement in them is just too powerful. If we try to force ourselves to sleep, it just makes things worse. So if you can go to sleep, you are already an expert in letting go. Now you just need to practice applying this skill in waking situations as well.
Jon Kabat-Zinn, Full Catastrophe Living: Using the Wisdom of Your Body and Mind to Face Stress, Pain and Illness
Art by Marina Grigoryan