I've come to call that "phantom lover syndrome". Spending a long time with some one, they really become part of you (and vice-versa), and not just metaphoricall

I've come to call that "phantom lover syndrome". Spending a long time with some one, they really become part of you (and vice-versa), and not just metaphorically. The brain keeps reflexively sending signals in that direction, and the void where there used to be a response can be very painful.