Drama where male lead loves the 2nd female lead and hurts the main female lead......the day he realises his mistake he is at the stage of losing the main female lead...

Any lang drama will do ....any genre except horror n historical 

You’re my destiny- hated it but you might like it

There was a drama that immediately came to my mind, and that is Fated to Love You. The main lead is obsessed with his ex girlfriend and really hurts the female lead in the process. Also this situation sticks quite a long time. You can also try You're My Destiny which is the taiwanese adaptation of the same drama. It's a bit older but I found it more intense then the korean version. I really loved this one when I watched it, was one of my first dramas. 

Another one that fits would be Full House. I'm pretty sure you've heard of this one. It's quite old but again a classic. I think it's the closest to what you described. The main lead is quite frustrating because he doesn't want to see the female lead for what she is hurting her in the process. Also he is quite infatuated with the second female lead for a long period of time. 

Goong is another very good example. The male lead is very indifferent and quite cruel to the female lead for a long time. He also always tells her how much he loves the second female lead a lot.

Another drama would be I Need Romance! The feelings come back just when I think about it. It's one of my favorites. There isn't really a 2nd female lead, more like a lot of other women because the main lead is kind of a playboy/jerk/childish adult. The drama itself is more mature then you typical one but i would really recommend it! Again their chemistry is off the roofs, and it's very nice to see the development of their relationship. Also in this, the drama starts with them already a couple so the image they have is not of a shy pair but more mature and sensual.

Those re the ones that come to my mind. Hope I was of help. I truly enjoyed all of them and recommend them eagerly!

the ones immediately coming to mind are

My lovely  Sam Soon

Ashes of Love

Ngao Asoke

Buzzer Beat is the one I'm currently watching and the first that came to mind

Gu Family Book

Rebel: Thief Who Stole the People

The Innocent Man - LMAO.  It's a lil bit different from what you're looking for but The 2nd Female Lead is the ex-girlfriend of Male Lead and she's also one of the antagonists in the drama. It's annoying because she kept on clinging on the Male Lead, and the ML keeps on meeting her whenever she calls.

The thing is, I don't think he considered it a mistake. It's a melodrama with a bit of revenge plot. Male Lead approached Heroine so that he can execute his revenge to the girl who betrayed him. The FL didn't know abt their relationship at first. Its a lil complicated. Its not exactly what you're looking for but It has those elements you mentioned. I do recommend it, lol. xD