Also, willows symbolize endurance. Remember the old story of the competition between the oak and the willow? The oak broke, the willow bent in the winds ( of adversity) and survived. If you endure the absence and keep hope "green" then mayhap your love will return. Just another possibility.

There's also the liminality of a willow. It grows on river banks half in the water half on the land. If your true love leaves you you're neither single nor wed. You're in limbo.