Plot is good, everything is good, the ending is well explained, nothing is wrong with this movie.
Just because the girl busted the handsome guy and loved a fat guy instead makes the movie bad.
Look, who cheated first? The fact that Jang Hyuk don't give the girl time anymore and always go hime so tired and then he let her coworker to flirt him. It's obviously his fault why the girl cheated her. The ending is well deserved, she shouldn't stay with that kind of man. They relationship is also hopeless, and I believe she will have a great relationship with the second Jang Hyuk.
If you hate this, you're just bad and you always want a good ending. Life isn't always full of sparkling joys, rainbows, butterflies, etc. People nowadays can't understand why a movie has a bad ending, they don't even use their brains to analyze the MAIN POINT AND MESSAGE of the movie.