Is it possible to be a Slytherin and yet still be insecure and doubtful of yourself? Because I know some who were sorted into Slytherin who are like that.

yes absolutely, slytherins can usually feel like they’re not good enough. like the recent image, they build confidence from the “outside, in” meaning they fool people into thinking they’re confident and well adjusted, even if they might be extremely insecure. eventually the idea is that they have confidence slowly begins to become more and more real.