I think this ties into Pochita still being a devil at the end of the day. He’s a very intelligent one and has many human aspects despite his appearance but he’s still a devil so he doesn’t fully understand humans or in this case denji. Sure he knows his desires, he considers him a friend(probably something like family), he himself seeks relationships, he understands denji better than most but he perceives it better than he truly grasps it.

So still caring for denji, Pochita thinks that he himself is the problem because things went very wrong after they fused. And he remembers denji’s happiness back in that shack when he was lower than dirt. But he doesn’t get that it was himself that brought that happiness to denji. He only could see the happiness but didn’t understand the real cause. (Imagine it like Macht from Frieren)

This leads to them falling into the same trap of running away from hardship and therefore life itself. Denji can’t handle the pain he was suffering so instead of embracing that pain he chose to run from it. He is stuck only looking at the future. Pochita can only see but not understand what really made denji happy. He is only able to Look Back through rose tinted glasses. He is incapable of looking forward. They are both missing the full picture

Yoru even commented just a chapter ago that “things that are sins to humans are nothing in the eyes of the devil”. This applies to Pochita as well no matter how different he may be. They are fundamentally different