drama where main guy falls in love with the girl who already in a relationship

Romantic Doctor, Teacher Kim

Check out:

• Loveholic - most like Chugakusei Nikki, FL is ML's teacher. FL has a boyfriend who is a cop
• Summer Scent
• Cloud stairs
• Secret Love affair
• In Time With you - leads are hbestfriends since highschool, and he has been in love with her for that long

Not really but maybe:

• Dream High / Long Vacation - meets with FL still single, but at some point of the story, FL gets into a relationship with someone
• Sangdoo lets go to school - leads are childhood friends/classmate, but the ML gets into trouble and is jailed, they meet again with FL becoming ML's teacher and already in a relationship
• Buzzer bear - the FL kinda is in a relationship with someone else. 
• I need romance 2 - leads are exes, and FL gets into a relationship with SML
• Discovery of Romance - Leads are exes, they meet again and FL has a boyfriend
• Angel Eyes - leads are childhood sweethearts that are separated, they meet again with the FL now with someone else
• On the Way to the Airport - both leads are married, has the same vibe/feel as Chugakusei Nikki of leads having strong connection and the melancholic feel of narrative
• Sprout - both the leads are in a relationship, and girl falls for guy first
• Proposal daisuken - fantasy element
• My Love Eundong and winter Sonata - leads are childhood sweethearts, and one got into an accident and was separated tragically (or thought dead), they meet again after a few years with one/both already in a serious relationship

FL is in a relationship when the ML meets her:

• +1 On The Way To The Airport
• +1 One Spring Night
• Before We Get Married
• Secret
• Ms. Perfect
  
• Mr. Sunshine - But iirc she had not met her fiance yet, at that point.
• The Crowned Clown - The ML takes the FL's husband's place.
• Extraordinary You - Her engagement is part of her character profile in the webtoon and not something she chose willingly.
  

FL is single when the ML meets her but later dates / gets into a relationship with / becomes engaged to / marries someone else:

• +1 In Time With You
• +1 Romance Is a Bonus Book - At the beginning of the drama she has already divorced her husband. We see the beginning of her friendship with the ML (before she met her husband) in flashbacks.
• +1 Dream High
• The Smile Has Left Your Eyes
• I Hear Your Voice
• I Need Romance 3
• The Imperial Doctress
• The Story of Ming Lan
• Story of Yanxi Palace
• Wednesday 3:30 PM - FL breaks up with her boyfriend at the beginning of the drama and the ML helps her deal with it.
• Moon Lovers: Scarlet Heart Ryeo
• Jealousy Incarnate - The ML knows the FL likes him and deliberately introduces her to his friend in a desperate attempt to avoid falling for her, but he immediately regrets it when they hit it off. 
• Kimi Wa Petto
• Empress Ki 
• The King's Woman
• My Fated Boy - The ML often acts as a shoulder for the FL to cry on over the years after her breakups. 

FL has left her husband / is divorced when the ML first meets her:

• Encounter
• Across the Ocean to See You
• Madame Antoine