A Bystander Loses Her Husband, Then the Protagonist Pressures Her to Marry

A quick summary: The protagonist meets his background-character lover, only to discover that the lover already has a family. He goes to great lengths to be the other man and keeps getting into hair-pulling fights with the ex-husband, without ever winning. Then he discovers he's won the jackpot, his rival kicks the bucket, and his lover loses his memory. He eagerly prepares to take the lover's place.

(I want to write some melodramatic, hair-pulling drama. I've recently fallen in love with the other-man trope...)

Message: Love should be right and unwavering