I'm looking for shows with very complex tortured love stories between the two leads. I've found that I really enjoy stories where the leads already have an established relationship either as friends or exes, like I Need Romance 2 and Discovery of Romance. I'm really loving the Unrequited Love (that hopefully becomes "Requited" by the end) between Kang Tae Ha and Han Yeo Rem.

I also really enjoyed Secret Love Affair so the friends/exes thing isn't really a requirement. I'm looking for romances between two people with an intense and complicated connection that for whatever reason can't quite be together. I'm also not opposed to love triangles as long as the focus is around the main couple. I don't really care for the meddling secondary female lead or meddling parents. I know many shows have the meddling character archetype though so that's also more of a suggestion than a requirement. In fact, I like dramas that focus mostly on the leads instead of constantly cutting away because of a huge cast of characters each with romances and back stories.

So basically:

- Couple with intense/complicated relationship, possibly friends or exes

- Less focus on meddling side characters

- Lots of longing glances and tension filled conversations

- Skinship: at least a little skinship is always nice, even if it's a flashback

- A happy ending is not necessarily a requirement but some form of closure would be nice. Just don't tell me beforehand if it does or doesn't have a happy ending. No spoilers!

- Can be a RomCom or Melodrama

- I looove Love/Hate relationships with lots of Beligerent Sexual Tension

I would really appreciate any suggestions. Thanks!

From your ptw list these are some that could fit what you're looking for, but only in the first two the leads have some connection from the past:
Will it Snow for Christmas? - leads are apart because of an accident that happened when they were teenagers
Missing You - same as above
Cruel City - she's an undercover cop while he's involved in drug dealing and gangs
King2Hearts - she's from North Korea and he's a royal from South Korea, which makes their relationship pretty difficult; they also have a love/hate relationship at first.

There's also The World That They Live In which is a bit difficult to put in a genre, because it feels like you're just following the lead's life, but they have dated in the past and seem more like regular people than your usual drama characters (which also reminds me of Discovery of Love and INR2). It's not that focused on the romance aspect though and there's a share of side characters, so I'm not sure if you want to give it a try.
Those all look great! And I was planning to watch The World That They Live in anyway, so it's good to know that it has the kind of romance that I like too. I think I may have found another one just by accident, Sang Doo Let's Go To School. It seems like it at least has the exes/flashback part that I enjoy.

Any reccomendations specifically for shows that involve exes relationships that are explored a lot with flashbacks?
9 End 2 Outs on your Plan To Watch seems to fit this pretty well.