Those dramas you regret watching so much that, if there is time machine, you would you go back in time to convince  yourself not to watch it
For me:

Nine : Nine times time travel (The worst Time travel i have seen in my entire life)
Room No:9

Secret

Fashion King (If you've watch this show, you know)

Scarlet Heart Ryeo (It was just so sad)

My Father is Strange (I don't like faux incest)

 Charlisa Ben:

Fashion King (If you've watch this show, you know)

Scarlet Heart Ryeo (It was just so sad)

My Father is Strange (I don't like faux incest)

i liked scarlet heart ryeo, it was good, kinda overdramatic and too sad towards the end, !
it dragged a lot too!!

Master Sun : at first i liked it so much specially when FL and ML team up for solving ghosts' problems but after EP 9 or 10 i lost my Interest for FL and MLs' relationship.the drama itself dragged so much for example FL decided to Travel another country for stupid reasons in last EP Secondary couple was only reason , that i could complete this drama 


Save Me : the rating is high and there were many positive comments and good reviews about this drama but i didnt enjoy this drama because FL was so Annoying and she always expected that ML and SML be there to save her from bullies

FULL HOUSE  FULL HOUSE  FULL HOUSE  FULL HOUSE FULL HOUSE  FULL HOUSE   FULL HOUSE

also,

Marry Him If You Dare, Romance Town, Sunny Happiness, Weightlifting Fairy Kim Bok Joo , Sassy Go Go, MISSING 9 (missing neurons more like)

I also didn't like Secret even though I like melodramas...

Melting Me Softly (I enjoyed all of the screenwriter's other dramas but this one was the worst) 

When the Devil Calls Your Name 

Biscuit Teacher and Star Candy (annoying love triangle)

Eulachacha Waikiki 2 (not as funny/memorable compared to the original season and I was upset that Joon Ki broke up with Soo Jin and got together with the other new girl

Leverage (for some reason it didn't grab my attention as much as Player) 

Oh My Ghostess (it is boring for me, I only continued because of Park Bo Young)

The Heirs (plot was meh, I just liked watching Kim Woo Bin's character the most)

Avengers Social Club (this also bored me, I don't get the high ratings for this one) 

The Cursed

Kakafukaka

Junichi

 Mask:

Master Sun : at first i liked it so much specially when FL and ML team up for solving ghosts' problems but after EP 9 or 10 i lost my Interest for FL and MLs' relationship.the drama itself dragged so much for example FL decided to Travel another country for stupid reasons in last EP Secondary couple was only reason , that i could complete this drama 


Save Me : the rating is high and there were many positive comments and good reviews about this drama but i didnt enjoy this drama because FL was so Annoying and she always expected that ML and SML be there to save her from bullies

i am on the same boat with Master's sun.
towards the end, kinda lost the point of the drama and dragging for nothing

 Sunset:

I also didn't like Secret even though I like melodramas...

Melting Me Softly (I enjoyed all of the screenwriter's other dramas but this one was the worst) 

When the Devil Calls Your Name 

Biscuit Teacher and Star Candy (annoying love triangle)

Eulachacha Waikiki 2 (not as funny/memorable compared to the original season and I was upset that Joon Ki broke up with Soo Jin and got together with the other new girl

Leverage (for some reason it didn't grab my attention as much as Player) 

Oh My Ghostess (it is boring for me, I only continued because of Park Bo Young)

The Heirs (plot was meh, I just liked watching Kim Woo Bin's character the most)

Avengers Social Club (this also bored me, I don't get the high ratings for this one) 

The Cursed

Kakafukaka

Junichi

don't even get me started on Secret, i started watching for the couple,.
they just dragging and dragging!!!

 Shishere:

don't even get me started on Secret, i started watching for the couple,.
they just dragging and dragging!!!

I don't get how they fell in love.. idk why but the drama just felt staged to me I didn't feel anything for it ?? 

and another one I regret was The Untamed special edition, I got bored halfway even though I knew it was like a giant recap of the drama. I didn't bother watching most of the last episode, just skipped to the ending xD I didn't hate the original version but a one time watch was good enough for me... 

Doctor John most poorly done and disappointing ending I have ever seen yet

Eternal Love  unpopular opinion

My Girlfriend Is An Alien The FL annoyed me too much to be considered cute or anything

Haha I don't know how you define "regret" but these two I regretted watching at the time.

Coin Locker Girl - I really hate horror and gore...Needless to say seeing my Favourite acting having his throat sliced open Scare the absolute hell out of me. I'm a lot better with horror now at least.

I also regretted watching Uncontrollably Fond. I cried super hard during the whole drama and then someone close to me was Diagnosed with cancer like the main lead, towards the end of the k drama.