In his dad's point of view, Louis is Nick's mistake. But for Nick, it is not. It's just that he follows his dad that's why he broke up with him. His dad wanted him to be perfect.
Oh right I'm just dumb sorry
Actually I read it twice to understand what Nick meant coz I thought the same when I first read it
In my opinion, Nick really loves Louis, that's why he said that he respects Louis by not telling him that his dad's thinks he's the only mistake he made. I think Louis might feel bad about himself if Nick will tell him the reason why he broke up with him. Maybe in the next chapter, Nick might feel jealousy towards Theo because he's free unlike him who needs to be perfect. That's why I also pity Nick because he's been through a lot just to be perfect in everyone's eyes. It's hard.
7 years passed and now you're realized it was a mistake? Dumbass.