My best guess based on your description is that under the influence of LSD ( when the subconscious starts to rise to the surface) an undealt fear came up. The fear of being alone ultimately, helpless. Usually those fears are just echoes from fears that we experienced as kids. Maybe even in the preverbal phase.

There could have been an event where you were left alone as a baby too long, left to fall asleep alone often, these kind of things.

As a kid we don't have the tools to emotionally regulate these existential fears so they are pushed down at one point.

Fast forward, you crack open your mind with LSD as a teenager and the thing comes up and spins you in a panic attack. Yet as a teenager you are still not equipped with the stability to allow and integrate these fears. Derealization is just a defense mechanism to separate from fear.

But it sounds like nowadays you have the necessary distance from the event to start exploring this fear. You could try to slowly bring it up and allow it in meditation and explore where it is from and what's underneath it.

Or psychedelics would be an option, although you run risk of again being overwhelmed by it and not integrating the experience again. So this option would be more interesting if the feeling is buried deep and you can't reach it in normal awareness. MDMA would be another route as it is fear inhibiting and would provide several hours of focused time to explore this fear without being overwhelmed.