I’d say that Ben is the type to suffer from a serious Inferiority complex, especially in situations where he doesn’t have access to the Omnitrix. 
I mean, he’s a fairly average kid, nothing really special except for the soccer mini retcon during Omniverse. He’s not strong, or tough, just an average kid. But he wants to be a hero regardless, and the watch allows him to do so.
But whenever he loses the watch, he suddenly feels completely useless, regardless of his prior achievements. He forgets all the times he rose up to be the hero, even when going alien was out of the picture at the time. Because I believe he subconsciously associates his worth to the alien gizmo attached to his wrist.
That’s partially why I’m so interested in Gwen 10 AUs, because then you get to explore how this affects Ben. Gwen was always a few steps ahead of him, so adding on the ability for her to become a superhero, something that Ben’s always wanted to do, could lead to some interesting writing potential. (Plus, Ben with magic sounds love the funnest possible thing.)