Dramas where the secondary love interest ends up with the FL in the end:
- Dream High
- Imperial Doctress
- Ms. Perfect
- Sound of the Desert
The closest thing I can think of to the ML ending up with a secondary love interest is maybe Gu Family Book, although her character is pretty obviously the FL from the audience's point of view. The ML likes another girl for a good portion of the drama and thinks the FL is a boy for quite a while (she cross dresses), so he doesn't see her as a potential romantic interest for a long time. She, on the other hand, has liked him since they were kids and recognizes him right away when they meet again.
Cinderella and the Four Knights, Another Miss Oh and arguably She Was Pretty have somewhat similar dynamics (the ML likes a different girl or doesn't recognize that the FL is the girl he likes, and he doesn't like the FL or doesn't see her as a potential romantic interest at first), but again from the audience's perspective the female character in question is obviously the FL.