I watched it while traveling on a plane 7-8 years ago, and have always wanted to rewatch it again due to nostalgia.
The film is about a female fashion designer who pursues a famous singer [or model?] (also her high school classmate), but her affections are not returned as he is a playboy. During a concert, she's backstage and drops a pen which causes the singer to slip and die. While making her getaway, she bumps into another former classmate who had a crush on her (Tech company CEO, used to be nerdy). The female lead never had any interest in him, but he persistently asked her out in the past. This time however, she accepts his invitation to dinner. Once home, she's depressed and goes on her computer, and a popup suddenly comes up. It asks whether she wants to go back and change the past, and she clicks yes. She is then transported back to her high school days (in her adult body), and lives in her former house pretending to be her younger version's cousin/aunt from America. For most of the movie, she works with the future tech ceo nerd in trying to get her younger self to pursue him instead of the playboy. During her time in the past, the female lead has slowly come to know all his good qualities. Unfortunately, her younger self chooses the playboy at the end after the playboy picks her as the queen of beauty at a school concert (talent show?). Afterwards, in the last moments of her existence, she is riding on a scooter/motorcycle with the future tech ceo and suddenly disappears into thin air. The movie then shows her younger self at various future points in her life talking into a walkie talkie given by her future self every time something happy/sad/monumental happens (I think the playboy eventually broke up with her). The last scene is of her same age younger self eating dinner with the tech ceo, where nothing happens between them. She thanks him for the meal, and as they are leaving he puts his coat in the wet gutter so that she wouldn't have to step into a puddle. As she walks away from him, he closes his eyes and counts to ten; hoping that she would still be there after he opens them again (this is something he does every time but he's always disappointed). This time his luck is good, and she is still standing there facing him after ten seconds is up. On that wintery street they can't stop smiling at each other, and the movie cuts to the credits.
(Awfully cheesy movie I know, but it seems so good when viewed through rose tinted glasses. Thanks a lot to anyone who comes up with the title!)