I'm going to share the one financial mistake that cost me the most money and what I learned from it.

In 2016 I had $18,000 in a savings account and $22,000 in credit card debt. For two years I kept them separate in my head — the savings were 'saved' and the debt was 'a problem to deal with later.' I was essentially borrowing money at 21% APR while holding savings earning 0.1%.

The math: I was paying approximately $4,620/year in interest while earning $18/year on my savings. Net cost of this mental accounting error: roughly $4,600/year for two years — about $9,200 total.

The fix was simple: use most of the savings to pay down the highest-rate debt, keep a $2,000 emergency buffer, and redirect the freed-up cash flow to rebuild savings while eliminating the remaining debt.

Mental accounting — treating money differently based on its label rather than its opportunity cost — is one of the most expensive cognitive biases in personal finance.