Ex gf/ second female lead

• The Story of Ming Lan - The FL warns the ML that the SFL is trouble. He's offended by her comments at the time, but later realizes the FL was completely right and regrets listening to the SFL instead of the FL when she warned him.
• Fight For My Way - For years the SFL came in and out of the ML's life and messed him up (and messes up the dynamic he has with his closest friends, including the FL) every time. One of the reasons he kept forgiving her was because he was touched by something he mistakenly thought the SFL did, when actually the FL did it. The ML comes to regret giving the SFL too many second chances and cuts her off.
• Tempted - The ML seduces the FL based on a bet he has with the SFL and SML. When the SML and SFL reveal the truth to the FL it obviously causes a huge issues between the FL and ML. The ML regrets his toxic relationship with the SFL and the bet they made / hurting the FL as a result.
• Moon Lovers: Scarlet Heart Ryeo - The ML and FL are in a relationship, but the ML is obsessed with obtaining power, ostensibly so he can keep the FL safe. The ML chooses to marry the SFL instead of the FL in order to cement his power. The FL chooses to leave the ML and marries his brother. Unbeknownst to the ML, the FL is not in love with his brother and is pregnant with the ML's child. The FL asked the brother to marry her so she could escape the perils and despair of the palace. The FL dies shortly after childbirth. When the ML discovers the truth, he deeply regrets his decision and behaviour.

My lovely Sam Soon is a classic case: he gets back with his ex for an episode or two. But the FL is not a bad person. 

My girlfriend is an alien. He gets partial memory loss, forgets the FL, but remembers the SFL, so he tends to believe everything the SFL tells him about the FL, at first. But he's a really smart guy this one.

If you like thai dramas, this is a classic trope in them. The 2nd FL will spew some lies about the FL and the ML will blindly act upon the lies and do real damage to his relationship. Examples of good thai dramas with that trope: Wong Wien Hua Jai,  Lying Heart, Game Rai Game Rak, Proong Nee Gor Ruk Ter

I am not sure bcz I watched it long time ago.

 If you are into BL then - Be loved in the house: I do and a chinese drama Perfect and Casual.