Going by the latest chapter, I think he realized he wanted to keep making cosplay outfits and stay with Marin, but he isn't ready to accept that. He loves his grandpa, and Hina dolls are what connects the two. He is trying to distance himself emotionally from Marin to not regret sticking with the Hina dolls. That's my assumption at least. I was worried he was just jealous, but thankfully that doesn't look like the problem.
I think that's just his rationalization. In reality, Gojo is being very irrational right now, and is trying to make sense of it. His attempt to fool himself also fooled you, hahah.
Judging from when this whole change started, it seems like, he feels a certain level of possessiveness or, exclusivity with Marin. He had some sort of special "relationship" with her, but never realized how much that meant to him. In fact, he developed romantic feelings for her, but unlike Marin, he refused to acknowledge them. This is because of his lack of self esteem. He doesn't want to ruin the "friendship" he has with her, and lose a person so amazing from his life. Besides, she "only came to him for the cosplays". So he buried the feelings away, and subconsciously channelled his love and passion for her, in his cosplays for her.
When she garnered so much attention from her latest cosplay, she reached a level of popularity from the people (namely men) around her. Fans absolutely adoring her. In the eyes of Gojo, a level of affection that he never dared show her, and did not know that he wanted no one else to do so either. Which is why he regretted making her so beautiful in her Haniel cosplay.
The scene of him being on the outside of the circle while she's encircled by men and women fawning over her, is symbolic. It symbolized how Gojo feels. Distanced from Marin, and that all this attention from others is going to cause him to lose her. Or maybe, he feels like he already lost her. That it's only a matter of time before someone better or "more deserving" comes along. He likely beat himself up about it, "reminded himself of his place"(his self esteem we see often kicking in again). Which is why we see this defeatist attitude, and subconsciously pushing her away. Because in his own mind, he already lost what he had. Or maybe, he deludes himself into thinking he never had it to begin with.
Gojo is not that beat up about the hina dolls.
Marin was his hina doll. he was already doing what he loved,
for the one he loved, even if he didn't know it. This was all just an excuse for him to rationalize his feelings. Because he doesn't believe in himself, and has most likely never felt this way before either. If a man with no self esteem is new to love and heartbreak, he'll do exactly as Gojo does.