He Replaced His Bride, So I Replaced the Groom

On her wedding day, wealthy heiress Vivian has her place as bride usurped by her fiancé's mistress-secretary. Utterly dead inside, she immediately grabs her childhood sweetheart — who hides his identity as the richest man — and flash-marries him on the spot. She then fully withdraws all financial backing from the scumbag, strikes back thunderously against the mistress's rumors, and leaves the scumbag utterly ruined, while she herself finds true happiness.