Letting go is a painful experience, and attachment, ego, or sunk cost: all play their role. We act like a fool and keep expecting things to work our way instead letting go.
All philosophies and religions like Buddhism, Hinduism, and Greek Stoics preached to us about it. All mentioned impermanence and the importance of living in the present.
Have you asked yourself how many days and hours you spent crying, getting angry instead of letting go of someone or something?