Spoiler alert
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He can't get over the fact that the MC no longer loves him or wants anything to do with him...
soon the bitchy lady (the one MC slapped) almost teams up with him/ex-fiance. He thinks if there is no King (current fiance of MC) then MC will come back to him. The plan was to spread a rumor that MC is infertile and that's why they broke off the engagement. (this would also imply that MC is no longer a virgin and give her a nasty reputation/unworthy to marry king).
Luckily in this situation Ex fiance doesn't agree to do it since it's just going to hurt MC...
BUT....
later
Ex-fiance ends up teaming up with the empress dowager/ former queen/queen mother? of King's country... (remember 1 of the conditions to being king's bride- have no connection with Queen mother)
the two of them: Ex-fiance and Queen mother = villains for the series

I'm starting to feel bad for him now. I still don't want him to end up with her of course, she deserves more than that. But I hope he isn't treated as a villain and starts acting like a mad man. Just want him to not be involved with her and have both of them live their own lives. He's not a bad person as I see, so I still want him to live a-okay life, not chasing after her when it's never going to happen.