Love @ Seventeen (2016)

Both main leads have parted/broken up with each other in the past due to misunderstanding. Now they are going to fall in love once again with the female lead having memory lost and they will try to clear up the misunderstanding. Both have second male lead who likes the female lead and protecting her after being hurt.

The girl MCs after experiencing hardships during their relationships in life had a total personality change. Through a series of events, they became seventeen again solely in mind. If you don't believe this was properly explained, you can read the summary. Love @ seventeen is a series whereas suddenly seventeen is a movie. They are both very similar but I cant explain without spoilers. Enjoy!

In both series after an accident the main girl believes she's a high school student.
Also her main interest tries to make her regain her memories.

Both male leads are transfer students and one party is treated as a younger brother or friend at first. Both couples spent a long time to get together after a series of misunderstanding but Love @ Seventeen took much longer and even advanced into workplace.

Has the same lead actor and actress.

Somewhat similar concept of time travel with the lead actress going back to her 17 year old self.

also has a bit of mystery and suspense in the story.

These two dramas are very similar. Both have the themes of school times, friendships, and romance. only difference is the relationship dynamics and the stories behind them.

Love @ Seventeen features a guy who spent time things about the girl and Fifteen Years features a girl who spent time thinking about the guy.

Both are about correcting mistakes of the past, they show how life choices can change your life but it's not too late for a second chance.