Sana's perspective of the whole story, hope you enjoy. Please forgive me for my grammatical and spelling errors 

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Sana knew it. She was even more aware of the mutual feelings Tzuyu was developing for her than the naive girl herself. As long as she was delighted that this very gorgeous and perfect girl was sharing the same kind of feelings for her, a growing worry planted itself at the pit of her stomach.

But the deeper she fall, the easier it is to hurt herself whenever she witness some of the most intolerable scenes. 

She almost couldn't explain how painful it is when she had to see Jungkook deliberately flirting with Tzuyu whenever she had to pass by the hall with all kinds of instruments huddled in her embrace after a whole day of hectic rehearsals conducted by her clubs. It didn't even help a single bit when Tzuyu kindly rejected the offer with that soft smile, that probably sent Jungkook's heart throbbing away from his rib cage. Sana could be this sure because even a simple smile from this angel could had similiar effects on her even though it was at a distance away and worse still, on someone else.

She thought it was the irritants dusting in her eye, but it turned out to be actual tears flowing down her cheeks. It was stinging and bubbling like acid burning in her heart, she was green with envy and over possessive when Tzuyu clearly wasn't hers at that moment. She knew just how much of a better person Jungkook is and just how much more he could afford to give Tzuyu, Sana couldn't offer Tzuyu the same deed even if she sacrifices her soul, so what makes Sana even worthy to like Tzuyu?

Due to the melting pain, Sana couldn't hold herself together anymore, she broke down in hysterical tears while crouching into a ball in the toilet after seeing the spams of messages flooding her phone's notification, her heart flinched in pain and she sobbed harshly when she recalled the horror the moment she saw the heart emojis labelled after 'Jungkook'. She almost fainted from all the exhaustion that day, she decided to get herself a distraction to ease the pain and that's how Mark came about.

And that lead to the confliction when Tzuyu confessed. Instead of the pang of bliss and warmth, all Sana thought was how terrible it would be if they were to be together. Even as friends, Sana couldn't control her emotions and often has to approach Mark for consolation whenever Tzuyu and Jungkook had the tiniest interaction. Sana chuckled bitterly and had pain filled up in her while she bluntly revealed her 'not so secret' admirer, she was blinded by the clouds of insecurity and down right rejected Tzuyu.

Sana had to forcefully convince herself that Jungkook truly fits her better, and also that they are not meant to be together, ever. Because Sana couldn't provide Tzuyu with anything much except for her