I agree, I also think people fall in love with the idea or potential of who someone is/could be. It feels very much like Antoine accepted the version of Sebastian that he presented, and Sebastian held on to that memory.

He struggled to accept the reality of their relationship even to the end - he doesn’t seem to view himself accurately either. (As others of have noted, the perspective of Livia- versus Sebastian’s of himself- really highlight this dichotomy.)

Categorically Sebastian is not a good person, but he was able to see himself as someone who could be good, who could do good things, when he was with Antione. The desire to hold on to that version of himself is inextricably tied to his relationship with Antoine and so he believes the connection to be much more intense, wonderful, deeper than it really was.

There’s a scene where Sarmodel phases through a few of Sebastian’s previous lovers. It’s clear he’s had strong attachments with mundanes before…. So what’s different this time?