female lead chooses 2nd male lead over male lead, but then they meet years later

Any shows where the male lead and 2nd male lead were in love with the female lead. She falls for the 2nd male lead and rejects the male lead. Years later, she meets the male lead again after breaking up/divorcing the 2nd male lead. She falls for the male lead while the 2nd male lead gets in their way and tries to get her back. The male lead and female lead end up together.

Maybe 

Delightful girl chunhyang - though she was blackmailed in breaking up with the ML

Goddess of Marriage - honestly it was kinda confusing who the male lead was, but i felt like it was LSW thus I think this scenario applies

Dream High - though they were not really separated for a long time. And people often said KSH was the 2nd lead rather than the male lead.

Not exactly in line with your description, but somewhat close:

• Sound of the Desert - FL meets the ML and SML in the desert on the same day and later on she goes to the city where they're from; they both look for her and are interested in her but the SML finds her first; she falls for the SML and rejects the ML's advances; the SML eventually rejects her because he doesn't think he can make her happy; later on she falls for the ML and ends up with him
• Imperial Doctress - I don't remember the exact details, but in the beginning both the ML and SML have feelings for the FL, she marries the SML but then the plot has her away from the palace where the SML is and together with the ML for a long time (in various locations), they go through a lot, and she ends up with the ML
• The King's Woman - the ML has been looking for the FL since they met in childhood; when they meet again the FL chooses the SML; the ML blackmails her into marrying him but gives her three months; she keeps her word and returns after three months; the SML and third ML get in the way in various ways but she does fall for the ML; however it ends tragically
• A Love So Beautiful - in the beginning the FL has an unrequited crush on the ML; the SML enters the picture and likes the FL pretty much right away; the ML slowly realizes he likes the FL but is very slow to admit it / shy about showing it; eventually the ML and FL date; the FL breaks up with the ML; he leaves and the FL low key dates the SML; years later the ML returns to win back the FL; the SML gets in the way a little bit

Twenty again -  the ML and FL are childhood friends (ml has a crush on fl, a serious crush) SML enters and marries the FL (all this happened before the first episode and was shown in flashbacks). The show starts from FL and SML in a phase of divorce. ML enters again, and at the end ML and FL end up together. SML does try to meddle between them but he was bit of a jerk (he too gets a happy ending with the teacher who loves him).