I don't think it is a 180 degree flip because he has been expressing that he is getting sick of his own lifestyle. So while it maybe hard for people to change and they tend to go back and forth, it isn't impossible with a good motivation and NB just might be it. In the webtoon, when he decided to change, he had his ex-girlfriend in his mind and not NB. So i believe the changes they made (including the personality of the ex gf) are pointing towards a redemption arc???
That being said, I do agree they need a time skip with separation to truly believe in their feelings for each other and are not driven by passion alone.