Old memories can actively block you from learning something new, not just fade quietly

When you've memorized one phone number for years, learning a replacement often feels unusually hard, and you're prone to blurting out the old one by mistake. Psychologists call this 'proactive interference': earlier learning doesn't just sit passively in memory, it actively competes with new, similar information. Benton Underwood's 1957 review showed the effect compounds the more prior lists a person has learned, helping explain why cramming similar material back-to-back often backfires.

— Benton J. Underwood, Interference and forgetting — Psychological Review, 1957