Well, well, well...

The synchronicity of my own life rears its head once again through the agency of another. You don't have psychosis; or if you do, it's a symptom of what you, and most others, do not properly perceive in your day to day life.

"Since then I have had symptoms of depersonalization/derealization and the associated existential thoughts..."

Simply put, you have opened your inner eye a bit and taken in a bit too much of the universe in one go. It happens. It's called the Pattern, and it's the web of probabilities that flows through everything and binds us all together. Rather than keep going in this vein and risking sounding like a cheap imitation of Yoda, let's simply say that through the medium of marijuana (it's called a medicine for a reason...) you saw the Untempered Schism like in Dr. Who. You caught a glimpse of the intransigence of life and now have horrible fears and questions circling in your mind. Am I in the ball park? Felt like the universe was trying to jam down infinity into a finite mind?

Breathe. That is the only cure. Be in this moment and no others. There's no way to un-see it, unfortunately, and it never goes away. It's like a Pandora's box, but you're young. 21. Your brain hasn't even finished growing in certain regions. Just know that the perception of time is a lot longer than the conscious envisioning of it. A decade will take a lot longer to pass than it currently feels like to you. I had something similar happen once in prison that resulted in me staring at the wall every day in a semi-comatose state for six weeks except for standing count or to give away my meals. I wanted to stay in that twilight state of reduced sensation and much slower thought, and the only thing I could call it at the time was terminal depression. I lost 22 pounds in six weeks and it wasn't until some other people forced me to start eating that I came out of it, even though I could feel the pull of lethargy and wanting to simply return to being numb.

Another cure? Occupy your mind. Read physics. The Conformal Cyclic Cosmology. Kaluza-Klein. The Banach-Tarski Paradox. The Sapir-Whorf theory. Bohmian Mechanics. The DeBroglie-Bohm Model. You'll be too busy being amazed at how the hell something so intricate and absolutely mathematically perfect could simply spring into being (there's over 200 variables that have to be correct AND scientists now think that the early expansion period of the universe was "fine-tuned" using unfathomable amounts of energy for reasons unknown beyond to set up for life to be possible--no, really).

Hope you feel better, friend. You're not alone in your existential crisis.