If you ask me what leads the most people to lives of mediocrity, it would be trying to justify "sunk costs." They are on the wrong path, but are desperate to salvage their investment of time, money and/or resources.
It’s almost a cliché today: “You’ve spent X years learning Y, and now you want to throw it all away for Z?” I know someone who hates his life as a doctor. But he keeps think about that 12 years he spent getting that degree. So he labors on, not conscious about the 40 years he’s losing.
There’s a reason they call sunk costs “sunk.”