Yep, I feel like this constantly. My default setting is that I feel like a poorly-disguised creature imitating human behavior, and I live with a perpetual background terror that at any second someone will see through it and call me out for all to see like the end of Invasion of the Body Snatchers.

Even the hypothetical/near-future/possible presence of another human being is enough for me to switch over into imitation mode. The only time I feel safe enough to drop the charade is when I am in a locked room/secure location that I have double checked to make sure I'm alone; then I feel like no one at all, which is sadly the most relief I've found thus far.