Chapter 1: Maybe have a former girlfriend accidentally meet the two while they were out and have that person rant about Joohyun's past? It would hurt Seulgi who may know or may not know Joohyun's past, but maybe it would really shock Joohyun into definitely and concentratedly making an effort to not stray and push Seulgi away instead in a bid to make herself distant and not worry about Seulgi's feelings. It would emphasize how different she is with Seulgi and make her realize that she cares and treasures Seulgi more than she lets on to herself. Reverse psychology might get her to stop fighting herself and actually give her all to loving Seulgi and proving it to her with her actions and dedication. Seeing Seulgi hurt due to her past actions would make her be reluctant to even attempt distancing herself from Seulgi because doing so would only do what she doesn't want to happen in the first place.

Joohyun's self-doubting whether she would be able to really make Seulgi happy and satisfied with her, so I think something needs to push Joohyun to realize her deeper feelings for Seulgi and that she is doing Seulgi wrong by even thinking about meeting with other women while she is with her as well the building of walls to keep Seulgi out. She already cares too much about Seulgi's feelings and welfare, which is something she should realize that the act itself means she actually loves Seulgi and is the reason why she is hellbent on not hurting Seulgi and wanting to be around her all the time.