You always make me happy

Yuna is miserable. She attended the wedding of her first love in order for her to finally let him go and move on to her life but right after she gives up on him, she gets transported back in high school just right before she loses her first love to another girl...
She is having trouble to choose... is changing the future will really make her happy but what if changing the course of the future become more complicated because the girl she hated most, the one who stole her boyfriend now falls for her...Will she falls for her as well?