"Because I'm weak" - No, bad author. You don't get to do this.
In what way is this guy weak? Physically? He broke a demon bird's beak with a single punch. Mentally? He outsmarted said demon bird who is a highly trained assassin. Emotionally? The guy had the resolve to fight said demon bird in the first place, knowing he would likely get killed and the thing is like 2 times his size.
Under no circumstance is this guy considered weak. I'd understand if he said "I'm not strong enough" and referred to the possibility of living life without his wife because that's a weakness I (and anyone really) can sympathize with. Perhaps that's what he was trying to go for and worded it poorly? I unno, it just feels like generic shonen trope talk. Decent chapters though, will see what happens next.