Dude loves her, she tells him "get better", he becomes a "get better bot" never feeling like he is good enough, to the point that he hurts both her and the rival dude that helped him early on with how good he became, yet still felt inferior.
Girl loved him, gave him a goal - why she didn't just accept him and worked with him together? No clue. He became good, his retarded way of going about this "bettering himself crusade" of his, started to hurt her. To the point she felt inferior to him so much that when she was supposed to go on a date with him after he won some prize - she was scared shit-less - near in tears - asking the rival dude what is she supposed to do.
Rival helped the MC. Then felt inferior to him - to the point that when the girl MC was "questing for to become better" came to him for advice he either fallen in love with her, or decided to make her his to fuck with MC. He managed to push her into a sex scene, she accepts, in-between the foreplay and actual sex she spots MC watching them - but she is broken to the point that instead of stopping the situation, she just accepts getting railed, and lets herself into the pleasure. Who knows maybe hoping this will somehow break the MC - pushes him away from their work so they both her and the rival cause stay together and not feel inferior. Or to stop MC quest and somehow fix their situation?
At the end MC is still broken - we don't know if girl and rival are together - doesn't matter thou - as MC would just see their relationship as a proof that he is still inferior to them. We don't really see rival dude, and the girl just averts her eyes from MC gaze. I can't say if its cause she still feels inferior/something, or she realized that she broken her love interest and made him a "better himself monster".