Slow romance for sure is a must or loads of sht that just keeps main couple from becoming together. Something where the lesser man ( in society views as in poorer or uglier, etc) gets the other girl to fall in love with him not on purpose, while the other girl is already in a relationship with a superior man (richer, more handsome, high standing, etc). This might be a bit specific but seems interesting to me.
Not exactly wit all the things you wanted but similar enough.
Lovers in Prague - Only think that is different from your preference is that the girl isn't exactly in a relationship but it can be interpreted like she is also.
Smile, You has a similar premise as you described but difference once again is that the girl isn't in a regular relationship with a "better catch".
This list doesn't include all your criters but I think you may like them though :)

Robber
Why Why Love
Big
Couple or Trouble / Fantasy Couple
Drunken to Love You

Enjoy
Devil Beside You A Love To Kill
lexies wrote: Not exactly completely by your specifics but similar enough.
Lovers in Prague - Only think that is different from your preference is that the girl isn't exactly in a relationship but it can be interpreted like she is also.
Smile, You has a similar premise as you described but difference once again is that the girl isn't in a regular relationship with a "better catch".


Thank you so much loved watching smile you I did drop at 23 because I got tired of the lovey dovey couple, but thats just because I usually get bored of watching couple progression. It was great while it lasted though very funny.
these somewhat fit, not sure if you have seen any sorry if you have ... pride kimi wa petto buzzer beat (they are both in different relationships) the woman who still wants to marry (not in a relationship anymore but her ex is trying to get her back while she is falling for younger guy) may queen (really long but the relationship doesn't develop until the very end) missing you biscuit teacher star candy (another younger man older woman story) dal ja's spring (another younger man older woman story)
I think this drama is accordance with all your specification
k-drama : What happened in Bali