One of the toughest lessons for me to learn in life is that of ignoring "sunk costs." Sunk costs are costs that have been incurred and which cannot be recovered to any significant degree. As applied to me, sunk costs are money, time, effort, etc. I have spent in the past that can no longer be recouped.
To ignore those sunk costs means to set aside the thought that "I've spent so much money" or "so much time" on acquiring something material or developing a project, because oftentimes those thoughts cause me to hold onto what I have to my own detriment.