Sumin has been concerned about protecting Sungji from getting hurt, from not making her cry again, but that's not what happen in the last two chapters. She's been treating Sungji badly without even thinking about her feelings at all. She promised Sungji in the beginning of their relationship that she will try her best to make it work, but one meeting with an Ex and she already loses her shit. That kind of behavious, the hot and cold behaviour will be confusing to Sungji and possibly hurt her.
Having Sumin as your very first partner is not something easy. Even if you're strong, it's not easy to deal with someone who is as messed up as Sumin and you will definitely get hurt. Being strong doesn't mean that you can't get hurt, it just mean you can handle the situation better and probably not give up on that person. That's what Seju has been doing for so long and hopefully Sungji will be able to do just as good or even better