I don't need to go on details but... she didn't admit that she loves Seju(She loves both!), but you're right on the part where she felt like she was the third Wheeler when the customers in the shop were talking about Seju's dad and Sumin heard it and became depressed and sad and Sunji saw that face that she made, and it stung her heart, so she felt bad and was all her fault for holding Sumin while Seju is idk xD anyways thats why she broke up with her... she needs to choose who. This is very depressing ;-; ヽ(`Д´)ﾉ
Love and be in-love are 2 different things. She told Sungji she can't ignore Seju because of their past and that's true. She was her very first love after all. She was not in-love with her though, not anymore and not for a long time. Sungji knew that. If Seju herself admitted she's not the one in Sumin's heart and Sungji can't get it then she really had to become stupid to deliver another drama.
Cause Sumin wouldn't stop thinking about seju when sungji and Sumin were in the cafe and sumin got pissed because those guys kept talking about seju so there you have it plus I'm surprised it didn't happen sooner with all the shit sumin pulled like when she cheated on her or when she slept in the same bed when she was told not to and sungji trusted her it was really inevitable but I bet they will be back together.
Calm down, if any of you know why she broke up with Sumin, can you tell me, because she might have a reason. When she was with Sumina, she looked really happy, but looking at Sumin, I did not see happiness just something like she did not want to be closer to her. After all, when Seju and Sumin are broke up, they still have sex. Everyone makes mistakes saying all the time that she is guilty is wrong, she should explain, but when Sungji saw Sumin and Seju in bed, having sex, Sumin try to explain that she had to do it, knowing that she has someone who loves her most. in the world. What the explanation is that, and later she admit that she still loves Seju, so maybe Sungji thought she is like a third wheel, maybe she thought if she be with her Sumin will be in pain