I'll share my two fav Chinese songs with you since the rest of the comments are all about korean songs hehehe :3

Kimberly - 爱你 (Ai Ni)
By2 - 你并不懂我 (Ni Bing Bu Dong Wo)

Yay! I love sad songs. Let me see if I can recommend some things. All korean though because I don't know any c-pop sadly. But here are my go-to korean sad songs. I feel like the best sad songs are usually by soloist so I hope that's okay and still suits your style ^^

Ali - I'll Be Damned (Like my favorite song ever~)
Vibe - As I'm Getting Older
Nevermind - Rain of Tears Again
Dalmatian - Hurt Me
Kang Seung Yoon - It Rains


And OSTs almost always have sad music ;;
I hope I provided something here you might like.

I love balad! So this was my list^^:
1. EXO- Don't Go (I prefer EXO-M)
2. EXO- Baby Don't Cry (I prefer EXO-M)
3. Seo In Young- Lets Break Up
4. Younha- The Real Reason Why We Break Up

I hope you'll enjoy it! :3

Seven years of love~
Kyuhyun sang it's cover too

infinite julia/omma/only tears

made me cry