We are not born separate. We learn it, the way we learn a language.
The idea that the self is an illusion stops sounding mystical once you watch a newborn, who has almost no boundary between hand and world, self and other. That sense of being a separate "me" is not switched on at birth; it is grown. The brain's default mode network, the circuitry behind all our self-referential thought, takes years to mature, and ordinary infancy is what trains it: this hand obeys me, that pain I can only guess at, so a line gets drawn and slowly hardens into fact. But a thing we acquired the way we acquired our first words is a fluency, not a law of nature. We are not less ourselves for noticing the wall was learned. What might soften if we held our separateness as a habit rather than a given?