I understand why most people are pissed at the ending. For some it's wrong for her to dump potato guy, For others Jae eon is a big no no and they (ML AND FL) would for sure suffer later on. BUT here's my point :
In last episode Professor said this to Nabi : EAT WHAT YOU WANT, DO WHAT YOU WANT AND LIKE THE PERSON YOU LIKE, THAT'S ALL YOU HAVE TO DO . For me this pretty much sums up all of Nabi's feelings. She wanted to be with Jae Eon knowing she might regret ( as she herself said in finale) because she chooses to be with him in the moment and not leave any regret.

Look man, Its simple if u want it to be . Everybody wants to be happy . She found her happiness in Jae Eon ; temporarily or permanent. That's the only way to find out.

Besides, it would have been very unfair to potato guy if Nabi had accepted him knowing she is obviously still in love with Jae Eon.
This drama served its purpose for me that is Yes I enjoyed it a lot.