Beyond Evil? hahaha, kidding. (But also, kind of not kidding.)

Honestly the only one I can think of that I've watched might be Strangers Again. But to be honest, I liked this drama more for the discussions with fellow Beanies than for it's actual content. Hot takes galore.

The Third Charm -- I think it's too raw and real, I could never watch it again. And The World of the Married, blood-boilingly frustrating and agonizing!

The Third Charm is really good and realistic.

The Smile Has Left Your Eyes will make you suffer a lot.

Que Sera Sera has characters who don't really know what they really want.

Call It Love.
My Liberation Notes. This one has three.
Strangers Again. Oooooh. This one really stretched it to the limit.
Search WWW. (One of the leads' relationship, I think Jeon Hye-jin's)
The Third Charm.
Queen Seon-deok.
Gwangaetto, The Great Conqueror (Damdeok x his first crown princess consort).

I'm Sorry I Love You: the OTP has so little precious time together, yet so much of it is wasted due to misunderstanding and noble idiocy. Hurts so good though...

I dropped Youth of May around episode 2, but I know this drama has the elements you are looking for, writer-nim.

OTP not together in the end… painful…

To be honest, the first drama on top of my head is Oh! Master. But the pain you might feel watching it will most likely be due to the poor storyline/script/directing. It was so bad it was excruciating to watch.

Sorry, this comment does not count as rec, but more of a warning.

I’m sorry Lee Min-ki and Na-na. I love you but…

Just dropping by to thank everyone for your recommendations - they all will be noted to make sure I NEVER touch any of these shows)))

Twenty-Five Twenty-One maybe. There is a huge uproar over the ending but I love it. I personally think it is the right ending.

I just watched Twelve Nights and thought it was pretty brutal on the agonizingly frustrating meter . . .

HI, BYE MAMA. You cannot root against any of the main characters. The sliver of hope was never meant to be for the OTP. The show ended in a proper way.

- Mr. Sunshine

- Memories of the Alhambra (depending on your interpretation of the muddled ending)

What Happened In Bali.
Just warning, it is an old school k-melo... with a brutal ending.

Moon Lovers: Scarlet Heart Ryeo
Need I say more?

Not counting Call It Love, I realize it's been ages since I last enjoy romance in kdrama. The one that comes to my mind is Princess' Man. It's achingly beautiful. I grew attached to their forbidden love story. Call me weird but I kind of love their fierce love and their struggle and the heartbreak. It truly makes a precious love story.

Gangnam Scandal...123 episodes of what you're looking for :)

Tree of heaven

Empress Ki has 2 agonizing love stories for the price of one.

Seven Days Queen (Pure agony with a more bitter than sweet ending)

Not a kdrama but the Taiwanese drama Someday or One Day might also fit the criteria. It is my personal fav bittersweet love story. Also the Chinese movie Better Days (another favourite) and the Japanese drama Soredemo, Ikite Yuku would be my other recommendations.

Interest of Love. Such a frustrating ending (for me).

Immediately, I thought of I'm Sorry, I Love You with So Ji-sub and Im Soo-jung. The title just screams agonizing love story. If asked for a top ten list of agonizing love stories, I'd probably just pick ten episode from that drama.

Que Sera Sera, also already mentioned by others here, is a good one, though the ending is open to interpretation.

And then there's A Love to Kill with Rain and Shin Min-a, which I don't remember as being good, but it was agonizing, and there was a memorable kissing scene.

Finally, the movie More than Blue with Lee Bo-Young and Kwon Sang-woo.

No, nope, never again. So triggered by this thread. A simple word analysis comes up with every term associated with abusive relationships. This written by someone who sat in a parked car sobbing to Aretha Franklin's "Never Loved a Man".
Time to change the tune to I Will Survive.

I don't think anyone has mentioned The Hymn of Death yet.

I think the 2019 dramaPossessed will be right up your alley.

TITANIC 😉

Are we going for melo without a happy ending? Or just no happy ending in general?

It depends who you ship, but if you found yourself rooting for the ‘wrong’ guy I’d say both ANSWER ME 1994 & ANSWER 1988 would be a torturous experience. Particularly Jung-hwan’s ‘confession’ will KILL YOU. (Did not include ANSWER ME 1997 cause if you rooted for the teacher, I’m JUDGING you so hard right now)

DREAM HIGH S1
They worked so hard to be the best idols in the industry but at what cost? Love?

My LOVE FROM ANOTHER STAR & CRASH LANDING ON YOU
Both had bittersweet endings, one falls for an Alien who looks awfully like Kim Soo-hyun and the other falls for a North Korean solider who looks identical to Hyun-Bin

MY FIRST TIME
ML doesn’t get the ‘girl’. It’s about their friendship and his unrequited love.

ARCHITECTURE 101 (film)
Oh boy this didn’t age well. There are a LOT of famous actors in this and it fits your description but ummm with a big caveat, it features a lot of rapey dialogue…

SUNNY (film)
I wouldn’t say it was a torturous love, but it was the female lead’s first ‘unrequited’ love. Anyway the film is all about female friendships and one of my all time favourites

20th century girl has all that with a nostalgia element, in a bite sized format because it’s a film.