Lover's Remorse
When you end up hating the person you liked/loved just the other day. You may or may not like someone else instead, and it may or may not be followed by a feeling of remorse/douchbaginess for getting over someone so quickly.
John's having a bad case of lover's remorse. Yesterday he loved Julie, and now he hates her.
Lovers Remorse
A feeling of regret after a relationship that did not turn out as you hoped it would. Sometimes caused by over - commitment.
She had lovers remorse after a one night stand and felt cheap.