Well, it's clearly your fault. 1 whole month without a way communicating to her made her love to you faded. In other hand, the impostor keep raising the flag as "The childhood friend".
The concept is great to be honest.
But, some people dislike how it looks (Including me).
But, if the author used it like this : "This is my incubbus form. It strengthen bla bla bla.". It will be better than this.
Well... I guess it's normal if the monster in your previous life is some kind of end-game monsters. But, really... you should have noticed it. You're a former great sage. Then, as a hero you exterminate all demons. The "normal" must have been degraded since then.