Eternal Sunshine of the Spotless Mind description:

Much to his surprise, timid Joel Barish is shocked to discover that the love of his life, sparky Clementine, has had him erased from her memory. As a result, hurt and angry, Joel wants to pay her back in the same coin, going as far as to undergo a painless but intricate medical procedure to do the same. However, poor Joel is utterly unaware that darkness is an essential part of the light. And as the once-cherished recollections of Clementine gradually fade away, giving way to a soulless black void, something unexpected happens. Now Joel has second thoughts, toying with the idea of stopping the irreversible process. Who said ignorance is bliss?