Thank you for your written report of your “first real taste of nonduality.”
As you stated: “It’s extremely simple…There are NO THINGS.”
Each and every thing (whether material or immaterial) is a creation of the mind. Were there no minds in the universe, to dissect it into separate “things”, there would be but a singular unbroken whole. Were there no minds, there would be no names given to supposed forms; there would be, simply, no thing (or nothingness).
Every form has a beginning and end; thus, is impermanent. Nothingness has no form; has no beginning or end; is formless. What you perceived, is the ubiquitous “shared nothingness” of all seemingly-real objects. All objects are dependent upon their formless actuality for their appearance All objects are limited, and appear “within” the unlimited formlessness; not one thing has ever been independent of it. All things, as you said, are “conceptual superimpositions” on a “universal” empty background.
Among the (immaterial) named forms is “consciousness” which, as you said, “creates” the relative appearance, the forms (naming each thing separately). So, even consciousness itself is actually another empty object. All things come and go; only the ever-present emptiness remains, forever present and unmoving.
So, there is no need to cling to or attempt to repeat this awakening experience. You have only to know that the underlying actuality, to all that we claim exists, is nothingness. One goes on living one’s life as an actor in a play: he knows that his “role” is unreal, and so he does not take his character (or that of others) too seriously.
And your fear is normal and natural, as well as temporary, when we move from the known (dualistic perspective) to the unknown (nondual perspective). When you recognize nothingness, you recognize that there cannot possibly be anything to fear in ultimate reality.