Usually i hate sad endings in manga but this was well done, much more meaningful than some idealised unrealistic "good ending" would have been.
It helps that i find the main character very relatable.
People are saying "brainwashing", "mindbreak", "training" etc. do you guys really think that girl would play these games if she didn't like "losing"?
I must say i expected Loser-chan to tell MC to go out with her if she ever won, i guess this works too.