I can understand a traumatized/abused person feeling anxious/comfused when suddenly showered with kindness, but this is hardly the first kindness he's received since his transformation.
As for his self doubt about his abilities, I could understand if he felt like he didn't "earn" them, but they don't say that, he just mopes and doesn't articulate *why*. It feels like he's depressed for the sake of being depressed.
Now, I *EXTREMELY* hate the trope of being oblivious to one's own strength in mangas/anime. I don't really find it funny beyond the first gag or two.
HOWEVER, in the case of trauma/abuse I can tolerate it lasting longer, but only if their mental state is properly articulated to the audience.
This manga is not doing the best job of that, but it's certainly better than several others I've read.