There are 40 more chapters to go, so I think there is plenty of time for their relationship to grow. They are already dating anyway. I don't think it was a question whether Sumin could really fall in love with Sungji. She is supposed to be the person who heals her after all.
I personally think the problem is in Sumin's character and the story's focus.
Sumin, despite showing that she cares about Sungji, her character still seems to not have grown much since the beggining. Being more mindful of Sungji and giving her three wishes is not exactly character development imo. She's just trying to treat her gf nicely-something she has done with her past girlfriends too.
On the other hand, she doesn't seem to deal with her past wounds at all. All she still does is going after pretty women and sleeping with them to try and forget.
Maybe now, she is starting to uncover her insecurities with Sungji, but I'm not sure that Gaji do this for character development, or they are trying to prolong the story and the anticipation of when Sumin and Sungji will finally sleep together (should that be the most exciting part of their relationship?).
The naration of the story seems to be relying quite lot on the physical pleasure for the readers, instead of exploring their feelings and situations a little deeper. I mean ok, they are all hot in this story, but after some point it's just not enough. (not that there is something wrong if someone wants to read wdtfs only for this)
And that was my problem with Sungji's character. She seems to be all sweet, lovely and beautiful. But not much beyond that yet.