The concept is indeed original from the ‘revenge’ topic that’s mostly written by people with screws loose in the head.
But it is still not anything better, there is nothing ’I moved on’ in this story, he didn’t move on he began a cuck and ended a cuck.
If a boy catch his girlfriend cheating (having SX) and does nothing, he’s a cuck, fair enough you’re not a violent person but still to do nothing at all… it suggests MC is someone with self-confidence issues, a background not introduced in the story, his sister’s death shouldn’t be the cause to that and his parents are clearly not bad either.
The changes of scenery are abrupt and lack any homogenousity possible.
The girlfriend’s motives aren’t said but just by the MC’s character it is easy to know why she cheated on him, in fact, if she didn’t break up with him was because of pity and if she came back was because of guilt, not love.
Last hope of salvaging this story and giving it at least a decent ending was for him to tide up himself and tell Minami to go with him and confront the vixen, rather than go with a white shirt and pants maybe even sleepers clearly showcasing a person that’s going through depression and clearly hasn’t moved on, you can see right there why she cheated on him.
Such encounter wouldn’t have only shattered the girl’s hopes (or pity) it would have given way to jealousy and it would have been the perfect revenge even if from his perspective, it’s something normal. Why is it wrong that you would ask your current girl to meet your ex to finalise everything? Not only would your girl trust you more as you’re giving this step with her but also he’d have his unintentional retribution.
But no, he showed up like a loser (And this is actually a realistic scenario, but not one that happens that often)
Anyway, interesting take but wrongly executed, finishing in 13 chaps is the best you could have done and although you said this is your own story, don’t expect my pity, I’m judging the story, not your life.