The synopsis reminded me of this Korean drama.

The main character, Gyung Hwi (28 male) is a university hospital resident. Back when he was in high school, he was bullied and attempted to suicide, however, Han Na Bi (a mysterious transferred girl) stopped him from dying. She ended up suddenly committing suicide after telling Gyung Hwi to live (not literally afterwards, but you get the gist).
For the past decade, he can't forget about her. However, one day, he wakes up to find himself as his 18-year-old self and it's a week before Na Bi commits suicide.
As the 18-year-old self, he was a loser, but he can't miss the opportunity to stop and save Na Bi from committing suicide. In order to do this, he has to find out what happened to Na Bi and the reason why she ended up killing herself, and of course, stop her from doing so.
Even though he is his 18-year-old self, he has the mindset and experience of a 28-year-old which will help change his gloomy past as he goes through his 18-year-old life.

"Will Gyung Hwi, who went back to the time he was an outcast, be able to save his first love and have a happy ending?"

I rewrote the synopsis from Dramalist's page about Somehow 18.