Spicy Pink would sort of fit. The Romantic Regret exists on two separate fronts.

First - The protagonists ex-boyfriend broke up with her 5 years ago because as a mangaka she was always busy. We later find out that additionally he was dating other women behind her back. They were "re-united" years later by accident when he was dating the younger sister of the protagonist's current boyfriend. The guy was convinced the protagonist still had feelings for him and proceeded to try and re-gain her affections (despite continuing to date her current boyfriend's younger sister).

Secondly - The protagonist's current boyfriend's ex-girlfriend came back into the picture. The protag was under the impression that the ex-girlfriend was dead (she was told so by the younger sister, who honestly believed the other girl to be dead). The boyfriend (who's a doctor -- plastic surgeon to be specific) and his ex were going out for five years, then the ex suddenly meets a rich, successful businessman and after secretly seeing the businessman for only two weeks (it might've been two months, I can't remember) she married the businessman and broke up with the doctor by breaking off all contact with the exception of a text message saying they can't see each other any more. Fast forward to current time and her marriage isn't a happy one, in fact her husband routinely beats her. Well out of the blue she starts calling the doctor and showing up at his home and clinic in order to try and reconcile.

It's a short manga (12 chapters), I don't know if that matters or not.

Wow! Lots of "romance regret" situations! This makes it easier to find as a lot of manga has this/these! Uhmm...

For the one-shots:
The first story (title one-shot) in S x M fits in a twisted way.
Nonchan to Watashi's got some messy misunderstanding...
The 3rd story: Wasurena Hill Bus Stop in Bus Hashiru kind of (at least you feel the regret).

As for full-blown manga/manwha:
There's some crazy regret from the second male lead in Do-Re-Mi-Fa-Sol-La-Si-Do.
Mars no Kiss (just picked it up yesterday night) has some regret going on (but only two chapters are out!! *cries)

I think there will be some regret coming up in Sakuranbo Syndrome -- this one's pretty low-key right now, but I think it's really good. Once you start the 2nd/3rd volume, you can't put it down. Nope nope... it's my (not-so) secret addiction right now. (Oh yes, the first volume seems ecchi, but it's pretty isolated to that first volume... I normally would drop reading anything ecchi, but this was done well.)

Sukitte Ii na yo has a chapter where a side character regrets being sex friends with someone. The story in general is really good. Shows a little bit of nudity (great, something for the supporters of that Tokyo law to gripe on.....but I digress)

Déjà-vu might work. It's not so much as regret as it is "I wish I could be with you but I can't". Still, it might work. Just so you know, there's four stories that are drawn by different artists but pertain to the same theme and story. Also, it's a manhwa.

It's Not Like That Darling has the male love interest in love wiu his brother's wife and regrets his jealousy and envy of his brother as his brother and the girl both died in a car crash. He can't seem to forget about her, and then. New girl comes along.

I'll try to think of some more later.

Would you like to join the Axis Order?

Pahanjip has a story about a noble who kills the husband of the woman he loves so that he can have her for himself, but she then does something that makes him regret forever. There are also other stories of romantic regret in this collection.

Avatar by tassyn.livejournal.com

Reading: Kingdom, Sangokushi, Historie

I read a review of Garden Dreams today, and noticed that it appears to have a lot of love-affair-related regret in it.

Hmm... I bet you already know about these, but I'll toss them up anyway.

Illumination, the title story -- main character regrets not being able to fall in love with someone.

The Cornered Mouse Dreams of Cheese plus the sequel -- jeez, the entire thing is made up of getting together, splitting up, and regretting what they do to/with each other. Lather, rinse, repeat. Except a lot more awesome than I just made it sound.

The secondary couple in Koori no Mamono no Monogatari has lots of this. Poor Wild is so hard on himself.

Kichiku Megane -- he regrets being a SICK TWISTED TERRIBLE PERSON.

I feel like Maiden Rose also has some element of romantic regret, but mostly it's just oh my god you people seriously need to work out your issues.

Ditto the above for Mirage of Blaze (novel).

You should really consider Honey and Clover.

I think that the series hints it, but it's a slow series. One of my favorites.

Zion no Koeda

Bukiyou na Silent

Lover's Doll

A Promise of Romance (Novel)

The Box

Why does only yaoi come to mind. orz

Lol @ wolf_unknown's rec of Kichiku Megane! Lmao.

And Blique, too. Believe me, babe, I totally get what it's like to have my brain yaoi-ed for the day. 😀

Thanks to everyone for keeping this thread going! Please keep recommending, because God alone knows I read manga fast enough to outstrip most Olympic athletes!

There's a lot in Manhuas Actually..
Just like...
Bringing the Nation's Husband Home

Kiri<3FansClub<3MEOWS=^•^=nyah!!