Funny thing is, Japanese have a bunch of good NTR stories but all of then are novels that still didn't get their manga versions. I read some of them in the last few years and some are really good, like Jinsei Gyakuten mentioned above (
tho that one got a manga now).
I still remember a good but short one, it was from the FMCs perspective the whole time. She was a flashy popular girl, and the MC was a gloomy otaku kind of guy who was into guitars. He promises her he will become successful for her, but she didn't believe him.
Then she got together with a nasty group on a concert and a playboy got her. She ended up pregnant, and got an STD during that time... I think she even left school.
We get a time skip, she lives a lonely life in a rundown apartment (
we also find out she aborted the baby and cured the STD). Her only comfort is listening to the music of her favorite artist, and guess who it is? The MC! During the time skip he became one of the hottest most popular singers in Japan!
She watches his latest interview and we are told that he never dated anyone since he loves only one girl and everyone of his songs is about her. Someone then knocks on her door... and it's him.
He finally found her and tells her he still loves her and as promised he became successful for her. She rejects him telling him she is too dirty for him, that she doesn't deserve to be by his side now, it would damage his reputation. He doesn't care, he will throw it all away if she won't be with him, because he only did this for her. She deeply regrets what she did and doesn't want him to stop his music, so she gives up and takes his hand... The end.
Not the best one, but the author did a good job describing her sad and regretful life, so i kinda wanted to share it. Don't ask about the name, i can't remember it to save my life