RESEARCH_FILE
The Sunk Cost Effect in Relationships
The sunk cost effect is the tendency to continue an endeavor because of what's already been invested, even when the investment can't be recovered and staying makes things worse. First documented in economic decisions by Arkes and Blumer, it was shown by Rego, Arantes and Magalhães to operate inside committed relationships: time, effort and money already spent push people to stay in unhappy relationships — and to reread an ended one as 'wasted years'.
How it sounds in your head
'I can't leave — we've been together ten years' and 'I threw away my best years' are the same accounting error at two moments in time. The years happened and had value either way; the bias is treating them as a bill that staying (or suffering) could somehow pay back.