I still struggle in this area but am learning more each day to be content with the life, the health and the things that I have. Letting go of control has been a very healthy exercise.
A few quotes from people that seem to know a bit about letting go.
The key to change... is to let go of fear. -Rosanne Cash
When I let go of what I am, I become what I might be. -Lao Tzu
Courage is the power to let go of the familiar. -Raymond Lindquist
Open your arms to change, but don't let go of your values. -Dalai Lama
Sometimes you have to let go to see if there was anything worth holding on to. -Anonymous
We must let go of the life we have planned, so as to accept the one that is waiting for us. -Joseph Campbell