The thing about this chap is Seju actually is letting go, she didnt hold Sumin back but at the same time, Sumin didnt stop being bothered by thoughts of Seju.
Actually, Seju has showed signs of letting go since chap 74, the talk about the house with Sumin, "if you hate my name that much, then you should leave by yourself", even though, in that talk, Seju seemed to avoid the problem and acted nonchalant xDDDD
She only picked up the will of clinging on after she moved in to Sumin's house, even right at that time, the bed talk, she already said this "Someday, we wouldnt be able to see you anymore, I'll pick up those memories and live with it". She clinged on but already prepared herself for that day. Seju knows she couldnt hold back Sumin this time around, she knows
I've almost understood Seju's actions for the whole manhwa, except only one scene, the scene with the new employee phone call when she repeated "nothing is gonna change, nothing is gonna change". What is not gonna change? Still a mystery to me.
That is the thing about both Seju and Sumin:
Deep down, Seju knows and realizes things but she always wittily and arrogantly appeared to be nonchalant and strong, she dodges the problem by her wit and words (the talk with Sumin in chap 74 and lunch with Sungji).
Sumin is also like that, what she feels and what she lets out in front of people might be the opposite.