Separation does not exist. It is part of Maya, the illusion of form. To escape, you must wake up. From a practical standpoint, feeling separate is a psychological state. You feel isolated and unloved, unable to connect with another person.
Separation is a valid experience.
Its reality status is secondary - a matter of belief. The point is, you feel bad, and that affects your mind, emotions, and body. It colors your actions and relationships. To escape, you must detach from your beliefs. Detaching from your ideas may sound easy, but it is not.