Ok here tanjiro is on a mission like any other when he meets muzan again and before they can actually fight muzan injects gim with his blood and skedaddles outa there. Now tanjiro doesnt go through the "pain" but instead while he is transforming his mind is fighting agaimst it so lile there is his friends, than muzan, tamayo and yuchiro, than lowermoon 1 and 5, the hashira, than uppermoons, basically his mind is trying to fight back but the demon blood is fighting aswell. The out come of this would be he remains human but he has a demon side to him that can come, like a personality disorder
oh yeah and they get closer ad time goes on so yeah
if you need a visual just think of naruto and how he fought his 'real self'