“I've already put so much into this, I can't back out now.”
What's happening
Money, time, or effort you've already spent is gone no matter what you do next — it can't be recovered by sticking with a bad decision. But backing out still feels like it would "waste" what's already gone, so people keep going past the point where it makes sense, just to avoid admitting the past spending didn't pay off.
A financial example
You've put $4,000 into fixing up an old car over the past year. The transmission just went, and a mechanic quotes another $2,500. The $4,000 is gone either way — the only real question is whether $2,500 more, right now, is worth it for what you'd actually get.
A better question
If I hadn't spent anything on this yet, would I still choose to spend this next dollar on it?