Just copying what I recommended from other subreddit. Here it is.

This film reminds me of Hana & Alice's best friend trope, btw.

For the first love (or kind of first love) trope with a bittersweet ending, here are some that I know:

• Be With You (both the Japanese & Korean versions!)
• My Girl and I (Korean ver.) / Crying Out Love, In the Center of the World (Japanese ver.) [This is the closest to 20th Century Girl. I prefer the Korean adaptation, btw. Still love it.]
• Love Letter (Jap.)
• Hotarubi no Mori e (Jap.)
• Our Times (Tai.)
• The Classic (Kor.)
• We Couldn't Become Adults (Jap.)
• Heaven’s Waiting (Phil.)
• Your Name Engraved Herein (Tai.)
• On Your Wedding Day (Korean ver.) / My Love (Chinese ver.)
• First Love/Crazy Little Thing Called Love (Thai)

While writing this recommendation, I realized that this trope was actually common with Japanese films back then too!