You forgot to say that Sumin and Seju have only one another, it's not like they just met. They have been together since the school days, been a couple. So it's not that easy just to end everything like that + remember Sumin's promise? To be Seju's family..it's super complicated.
Totally agree with you. I think Seju not confident enough to love someone. She has a great background, sexy, beautiful and smart but true love. I think Sumin and Seju are dependent on each other. But Seju doesn't say she loves Sumin by a normal way and Sumin hate (with love) her. Sumin always try to find a true love or one woman can satisfy her (sex, feeling) but only Seju can stay to the end like Sumin said in chapter 14.