Hello,
im looking for some dramas where the girl falls in love with someone who just sees her as "dongsaeng" and who is already in love with another girl. 
So basicly just a drama about a female lead dealing with being in the friendzone and maybe still trying to win her crushs heart. You know? Could be K, J or Tw-Drama. Thanks in advance! :) 
There is a japanese movie called Her Senior.
Already seen this one :( Thank you anyways!
Some J-dramas (though they don't totally fit):
Summer Nude (not the best drama ever), Shitsuren chocolatier, Tsuki no Koibito, maybe Yamada Taro Monogatari.
Hope it could help.

Maybe "Tomorrow's Cantabile" if you are looking for a KDrama, but it also does not fit perfectly ;)
As for Chinese Dramas, "You are my Sunshine" kind of fits (concerning flashbacks, which take up a lot of the drama)
Heroine Shikkaku (jmovie)
Ex-Girlfriend Club (kdrama)

P.S. I think I already recommended EGC to you once before :P
protect the boss -  the second female lead got friendzoned twice, its a complicated love/friendship square.  even thou shes the second female lead, you will root for her.  she's not your typical second lead  :)
A little old but I think this matches the most - Snow White
Heart to Heart
Thanks! :) Ill will look them up!
was going to suggest You're Beautiful, but you've already seen it :)

Doctor Stranger
My Amazing Boyfriend
Love Letter k-drama. He friendzones her for the church. All of the Hong sister's earlier work had the female lead falling for the guy first.