I'm only on episode 9/16 as I'm writing this review. Originally, I was going to wait until all the episodes were released until I started watching this drama. But I kept seeing so many clips that I just had to start. The beginning started off a little slow and now I've just spent the whole day binging the show. This is such a good, refreshing, and touching drama. I love the way its shot and I love how we, as an audience, is reflected as Min-Chae. For example, when Hee-do said she had a boyfriend, I was like WHAT and it immediately cut to Min-Chae with a similar reaction. There has been so many moments of this drama already where I felt so touched and moved to tears. I love watching the characters embark on their journeys, learning painful lessons and growing from them. It's exactly as Yijin said, they're going through their growing pains. I can't wait to see how the rest of this drama unfolds and I really hope there's a very happy, very cute ending waiting for us at the end of this journey.

EDIT: Coming back after finishing the rest of the drama
I knew it was coming, and I knew it wasn't going to be a happy ending but I really had high hopes for a happy ending anyways. Honestly, I'm not too mad about this ending but I definitely don't think this ending was done as well as the rest of the drama was. I understand that the writer wanted to portray a realistic ending, but I didn't think the reason for the breakup for that justified. I definitely think that it was a bit out of character for Na Hee Do and Back Yi Jin to basically give up on their relationship. The break up itself was beautifully done and really captured the pain of a break up, but I just wish the reasoning behind the break up made more sense. In addition, i didn't even know Na Hee Do's husband is someone working overseas, so it seems even more ridiculous that Na Hee Do would break up with Back Yi Jin because she didn't want to always be waiting for someone and be in misery.
I didn't think too much about the present-day Na Hee Do and Kim Min Chae, but after reading other reviews, I agree. The present-day characters basically served no purpose to the show and only teased the viewers. I was also annoyed because they included that scene where present-day Na Hee Do forgot about the their first beach trip, but the scene made it seem like something happened to her to make her forget, instead of her just forgetting because it's been so long. I feel like it makes more sense that once it was brought up, she would fondly but faintly remember her friends and the memories that they once shared.
But overall, the drama was still emotional, impactful, and really captured the exciting but turbulent time of youth. I don't regret watching this drama but I really wasn't expecting the emotional damage this drama has given me.