Mark :
It would hit him like a truck and he'd be out of it for quite awhile. Mark would have a really hard time telling you this as well. He'd hate to see your feelings be hurt but he had to be honest.
Taeyong :
This would tear Taeyong apart because he still cared for you so much although he didn't love you the way he used to. He'd be one to try and push away the reality of losing feelings.
Lucas :
Lucas would laugh through the pain. Which sucks because no one would really notice how sad he was. You'd be able to know there was something wrong and when he'd finally tell you it would be like he exploded with this thing he'd been hiding for so long.
Jaehyun :
Tries to be as kind as possible with telling you this. Easier said than done but he never wanted to cause you any pain although this was the worst band aid to rip off ever. He might have a hard time truly letting you go as well and would check in on you ever now and then.