The short answer
If the same old memory keeps returning instead of fading like most memories eventually do, the reason is usually less about the memory and more about what happens the moment it surfaces: repetitive, passive dwelling on it — what psychology calls rumination — which research shows actively prolongs distress rather than resolving it, and can make the memory itself more, not less, likely to keep recurring.
The research
Foundational research on response styles found that people who respond to distress by ruminating on it repetitively remain distressed significantly longer than people who use distraction or another response — meaning the act of dwelling on an old memory can itself prolong how long it keeps recurring.
— Nolen-Hoeksema, S., Journal of Abnormal Psychology, 1991
Rumination keeps the loop open
Foundational research on response styles to distress found that people who respond to a bad feeling by dwelling on it repetitively stay distressed longer than people who distract themselves or take some other action, and that this pattern specifically prolongs the episode rather than working through it. Applied to an old embarrassing memory, that means replaying it in your head isn't neutral — it's often the exact mechanism keeping it live years after everyone else involved has forgotten it.
Why this particular memory, still
Memories connected to strong negative emotion tend to get pulled back into awareness whenever your current mood resembles the mood you were in originally — a documented pattern in memory research known as mood-congruent recall. A bad day, a similar social situation, or even just feeling slightly insecure can act as a cue that reopens an old cringe memory that a good day wouldn't have touched.
Breaking the loop without fighting the memory
Since rumination is the mechanism doing most of the damage, the more useful move usually isn't trying to erase the memory — that rarely works — but interrupting the repetitive replay early. Noticing it's happening again, marking it, and redirecting attention tends to shorten the loop far more reliably than arguing with the memory's content each time it resurfaces.