What does ‘eternal sunshine of the spotless mind’ mean?

Here is a small excerpt:

How happy is the blameless vestal’s lot!

The world forgetting, by the world forgot.

Eternal sunshine of the spotless mind!

Each pray’r accepted, and each wish resign’d

In Pope’s poem, Eloisa is in anguish over the powerful feelings she still has for Abélard, especially in her dreams, and by the realization that, now a eunuch — something he regards as a mercy that freed him from the “contagion of carnal impurity,” — he could not return her feelings even if he wanted to; and so she begs, not for forgiveness, but for forgetfulness.

Basically, she wants to forget him. And she wishes for the Eternal Sunshine of the Spotless Mind.

If he is gone from her thoughts, she will be happy again.

It also explains the premise of the movie with Kate Winslet and Jim Carrey.

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What does 'eternal sunshine of the spotless mind' mean?