Even if he is her first love, this is a matter of self respect. She's literally letting herself be dragged around by Eun Hyuk emotionally and mentally. I know how first love can be, but after a while you have to prioritize yourself and your dignity because if you can't respect your self, no one is going to do it for you. I used to be the biggest EunHyuk supporter, infact my acc name used to be "GoEunhyukFangirl" but after everything that has happened, I just can't support or root for him anymore

Girl? It's really not that fricking complicated, guy 1 ghosted you for years, came back randomly, started meddling in your life whilst still drawing a clear boundary, didn't give any explanation for is action and refuses to own up or take any responsibility, on the other hand guy 2 is madly in love with you, stayed with you always, prioritized you, showers you with attention and affection, yearns, owns up to his mistakes and apologizes, is a literal walking green flag.
How is making a choice any bit "complicated"?