Just because you love somebody doesn't mean that this love you have for them gives you infinite strength to deal with everything that comes your way perfectly and just like you should and without getting your own wounds. Seju suffered the entire time throughout her relationship with Sumin, because she was living with a terrible family, who hated her guts and forced her into positions she never wanted to be in. How did she create this mess on her own? Her family hated her for reasons she never had anything to do with at all. It hurts dealing with things like that and after a while, it just robs you off of your energy. Sumin has always been Seju's escape and just because Seju has always had a rather unique and vague way to express her love for Sumin (which is probably simply because the treatment her family gave her had left some noticeable traces on Seju), the simple fact that they had been together for so many years and that Sumin never doubted Seju's feelings for her can only mean that there wasn't a time in their relationship, in which Seju didn't love Sumin. The fact, that it seemed like their first time sex together wasn't special to Seju can, in my opinion, either mean, that by that time, because of what her family continued doing to her, she was emotionally already F'd up enough to really not care that much, or she was acting. Sumin was obviously nervous and maybe Seju was just acting like it's all no big deal to not freak Sumin out even more.
Anyway, the point I was getting at is that love doesn't give a human an infinite amount of patience and strength. That's why breakups happen in the first place. Even while Sumin was depressed, Seju still had her own sh*t to deal with. Now Sumin was suffering, which probably hurt Seju as well, and Seju still had the patience to try for a whole month to carefully get Sumin back on the right track - to no avail. So there she was, with a pathetic excuse for a family making her life a living hell, facing a future she never wanted and having to deal with the hurt of knowing that her beloved was suffering and with the fear of probably losing Sumin for good. Humans can break, you know. At some point, they just reach that point, when the weight they're carrying just all gets too much and it crushes them. Worst part is that it doesn't happen one by one so that you could at least try to keep it together, no. It happens all at once. Bam! Just like that. One second you feel like you can still bear this if you just keep your head above the water somehow, then a moment later the thoughts come flooding into your head, your heart starts racing like crazy and you basically just see your entire world crumbling and falling apart right in front of you. It's like a punch in the face that hits you out of nowhere and it leaves you distressed, disoriented, vulnerable and all in all extremely prone to doing something extremely stupid. Prime example everyone knows: Britney Spears.
Honestly, I really don't think Seju cheated on Sumin because she didn't care about Sumin anymore. Or maybe at that point, she actually really didn't care anymore about Sumin as well as about herself or the entire rest of the world. She was probably just like "F*ck it all!" and ended up doing what she did, because she was yearning to at least feel something good. That's my view on what happened. Seju reached her breaking point and did something extremely stupid, which she still regrets until this day. Everybody makes mistakes. And it's mostly when we're emotionally at our worst that we do the biggest, stupidest ones.
Like I said, I'm not blaming Sumin and I understand Sumin's view of it all as well. Really, I do. What I said about love not giving a person an infinite supply of patience and strength goes for Sumin as well. She was relying on Seju to be there and ended up getting disappointed in the worst imaginable way. I understand the rage she felt. After seeing Seju in bed with someone else, I guess Sumin felt the same things I think Seju felt when she thought there was no way she could help Sumin and that she'd lose her. Seju really isn't a position to ask anything of Sumin and just because she too did her share, that ultimately led to their catastrophical breakup, I don't think there's anything Sumin owes to Seju. I just really do hope that one day they'll manage to talk everything through and maybe turn their messed up relationship into a normal, healthy friendship. I'd love to see that happen. ^^