Hi! 

I just finished watching Moon Lovers and I liked the part where Wang So had so much regrets (like not seeing Haesoo before she died, not reading her letters, and not knowing that he has a daughter). Call me savage but I really like it when the lead (male or female) really feels bad lol Can you guys recommend me more heart-wrenching dramas though it would be better if the ending is a happy one. Thanks a lot!

Hi!!! :)
Lol savage...just kidding!! :D  Well I'll recommend a few.. :)

http://mydramalist.com/5957-drama-special-series-s...
http://mydramalist.com/4360-drama-special-series-s...
http://mydramalist.com/7116-limit

The 3 above don't have romance...it's regrets between brothers/friends/classmates...not between lovers.

http://mydramalist.com/7028-two-weeks  ----> another drama with Lee Joon Gi! :)) Here too he has many regrets!
http://mydramalist.com/81-autumns-concerto
http://mydramalist.com/288-49-days-2011

I hope you enjoy watching them!! :D 
Savage indeed ^^ Thank you so much! Can't wait to check them out especially Two Weeks :)
:D :D  You're welcome!! I hope you enjoy watching them! :)
I love Two Weeks...that's when I 1st saw LJG and fell in love lol