Need recs for dramas where the lead has some psychological ability or disability, such as multiple personality disorder, no long or short term memory, perfect memory, face blindness, social disorder, hear thoughts, etc... I'm not looking for a too serious or sad drama or a drama which is mainly about a mental illness. I prefer if the psychological problem is there to give the lead a sense of uniqueness or specialness rather than a drama that revolves around the problem. Also must have happy ending and romance. 

I've seen these: 

Flower Boy Next Door - social problem (2.5/5)

Good Doctor - autistic (3/5)

Heart to Heart - fear of people (4/5)

I Hear Your Voice - can hear other ppl's thoughts (5/5)

It's Okay, That's Love - schizophrenia (2/5)

Kill Me, Heal Me - multiple personality disorder (3/5)

Protect the Boss - i don't remember exactly what his problem was.. he couldn't speak in public or something. (3/5)

Rich Man, Poor Woman - face blindness (i think) (2/5)

Girl who sees smells - girl can see smells, villain has face blindness (5/5)

Once Upon a Time in Saengchori - Features a math genius who loses his ability and more - he becomes absolutely incapable of understanding numbers. 1+1=? He wouldn't know. He wants to hide this little problem of his and therefore transfers branches in his company, ending up in the poorest performing team of the company (relocated into a countryside village in hopes of them all resigning). There will be romance and the ending... The ending is both happy and sad at the same time. This drama has that fine balance of happy comedic things coupled with suspenseful murder mystery.
hmmmm... maybe these?
Pinocchio - girl hiccups when she lies
Me Too, Flower - deals with depression
Secret Garden - male lead has some psychological issues (all i can say without spoiling)
Yamato Nadeshiko Shichi Henge - female lead has fear of bright/'shining' things, loves horror. male lead antisocial.
other than what was already mentioned id suggest this one:
mask - male lead deals with ocd. ( and not actually mentioned in the drama but it seemed that he also has ptsd) 
the drama is also classified in the psychology genre too so...you'll definitely be hooked.  

Falling for Innocence - The guy suffers from cellular memory syndrome (he gets the feelings from the heart of his donor).
Funny and sad and there's Jung Kyung Ho.
Flowers for Algernon or Hello, God - it has some sad moments, but nothing too heavy
Satorare - the main lead is a 'satorare'; it's a special condition that enables everyone to listen to his thoughts
Ataru - a genius man with autism
Aozora no Tamago - deep anxiety and trauma, but it's heart-warming
Blade Man - main lead has a special condition that makes him grow blades when he's angry
Dr. Frost - lost his emotions
God's Quiz - suffers from a rare disease
Hyde, Jekyll, Me - split personality
The Plan Man - it's a movie, but it's very good: the main lead suffers from obsessive–compulsive personality disorder

I hope you'll like them! ^^
i remember you , deals with serial killers 
ST Scientific Taskforce (with prequel and movie)- the main plus several supporting characters have various psychological issues while they try to work together in a special investigations squad. as for the romance dept... well some can argue that the two main male characters have a thing for eachother jk
Date- male lead is a hikikomori
Thanks! Watching Mask right now. Will check out your other recs once I'm finished.
there's Hello, God too!
Could only think of 2 more;

Arang and the Magistrate
Oh my Ghost(ess)