ML is still in love with his ex BUT is married/dating FL

 Ichika Kaneki:

dating, married anything. I want to watch a drama which has a story like Joji's Glimpse of us. 

ML is dating FL but he is still in love with his ex gf. what new i want is him to fall for his current girlfriend after realizing her value. 

The ex can be the FL(as per screen time) or an important supporting character as well i dont care but ML ends up loving the person he is with , aka current girlfriend/wife or anything.

Fits your description:

• Mr. Queen - The SFL isn't even the ML's ex yet in the beginning. When the ML marries the FL he's still seeing the SFL and still intends to marry her as a concubine if he can't marry her as his wife.

You might also appreciate: 

• The Story of Ming Lan - The SML and his wife, not the ML and FL. 
• Itaewon Class - The ML doesn't start dating the FL before falling for her, but there's a significant period of time after the FL confesses her feelings to the ML where he rejects her / is still hung up on his first love ("almost girlfriend" would be more accurate than "ex girlfriend") but he becomes very close with the FL / treats her as his right-hand-woman / doesn't realize he's falling for her.

In the ballpark: ML has feelings for the SFL when he starts (fake) dating the FL, but he ends up catching real feelings for the FL:

• Tempted
• Wednesday 3:30 PM

And just for fun, the exact opposite: the ML has feelings for the FL before he starts dating the SFL, but (consciously or subconsciously) he avoids confessing to the FL because he doesn't want to ruin their friendship / lose her:

• In Time With You
• Kiss Scene in Yeonnamdong

Fated to Love You he gets with her out of spite basically

Go Back Couple though it's more nostalgia for what could have been. Familiar Wife is a more hardcore take on the trope. 

Love in Sadness the ex is an ex wife but she doesn't really get between them

My Lovely Sam Soon oldie but goodie. There's also a thai remake but the original version fits more with your description.

Nakark Kaew very similar to Love in Sadness. Sud Sai Pan as well.