Can you guys suggest some really really sad series or movie that can make me cry a lake? Please ㅠㅠ 

hmm any specific genre? Anyway i'd just list whatever series/movies that have made me cry:

Movies:
- Wedding dress (my favorite! mother-daughter relationship)
- Werewolf Boy (romance, fantasy)
- Miracle of cell no. 7 (father-daughter relationship)
- Sunny (friendship, youth, a lil bit comedy)

As for drama i think i've never watched a pure sad series, so i dont think i can help you on that one. Hope it helps btw! :3

 
I legit cried for half of My Annoying Brother... however, I am a bit oversensitive lately so im not sure if it was really that sad :D

As for drama i remember crying a bunch during first 10 episodes of Another Miss Oh. It wasnt /sad/ but I pitied the main character so much. she was truly the most unlucky person so if you find it easy to sympathize with drama characters you could try this one :D
Boku no Ita Jikan - the entire story is just very sad and heartbreaking
Babo: Miracle of a Giving Fool - there is a tragic twist at the end that just has a very big impact on you
Ugly Alert - very long drama, but it has one of the best stories out there; it's just so full with emotional, genuine moments and it has both heartbreaking and sweet scenes! If you have time, you should really check it out
Uncontrollably Fond - I think the whole story just added it up to that finale episode because it made cry rivers!
Empress Ki was a bit heartbreaking for me.....I definitely cried

Marriage Contract is another sad one
kill me heal me (kdrama)
secret (kdrama, 2013)

both are really sad but secret is a lot darker
ditto to finale episode of Uncontrollably Fond :'(

Blossom (movie)
Heart to Heart (drama - more so in the second half of the show)
Gatecrasher (special)
Glass Prison (special)
Secretly, Greatly (movie)
Scarlet Heart Ryeo hands down
Tree Of Heaven
I'm sorry, I Love You is so sad. I stopped watching it, because I can't watch it anymore as how sad it is. Keep the box of tissues close.
Missing You
It's Okay, That's Love
Cruel City
On The Way To The Airport
I'm Sorry, I Love You
Fantastic
The Full Sun
Can You Hear My Heart
Maids
The Princess' Man
That Winter, The Wind Blows
Cain and Abel
Innocent Man
Thank You
Heart to Heart
Scent of a Woman
Marriage Contract
kill me heal me

More Than Blue
These are the one's that at some point have made me cry -

Series:

Empress Ki
49 Days
The Princess' Man
Secret
Who Are You 2013
Wonderful Life


Movies:

The Last Princess
A Werewolf Boy
Orange [Japanese but a sad and beautiful movie]
Koizora [Japanese]
Time Traveller [Japanese]
It's a bit old but many of my friends cried a lot for "I'm sorry I love you". I didn't watch it,though.
My drama suggestion would be,

Moon Lovers:Scarlet Heart
Kill me Heal me
Master's Sun (again this might not be what you're looking for but some certain scenes are so overwhelming that I couldn't stop crying when I watch.)