Sumin is the one who deserves better, she's in a relationship with a girl who never bothered to know about her bcs shes scared she gonna feel insecure as it was evident when Seju told Sungji that Sumin and her where a couple.
Sumin needed someone special who won't fail her during her hard times, but Sungji failed her when her problems with Seju surfaced and left her to protect herself and only took her back when Seju disappeared. She was being selfish and cared less about Sumin's trauma, that she has trauma she has to overcome to heal. After this Sumin was never able to open up again to Sungji about her problems as it was shown in will spin off bcs Sumin developed a fear of abandonment so she's scared to open up to her partners about her problems bcs shes scared that they will leave her just like Sungji did.
Sungji didn't sacrifice anything. When problems happened she backed off to protect herself bcs she was deeply insecure. And only accepted Sumin back when Seju who made her feel insecure left.
Sumin is the one who deserves better. She has too much trauma but too bad not a single partner of her helped her to overcome, none of them cared enough to have an emotional relationship with her and make her feel comfortable enough to talk about her trauma. All what her girlfriends cared about is sex and the fact that Sumin is rich with a pretty face.
Were a couple*
The blonde deserved better she was just a pushover, black haire fl was bad she was just hurting blonde in every way keeping secrets from her cheating on her and much more i truly wish she'd break up because blonde was always giving and sacrificing on the other hand black hair fl was just too selfish