Couple of things and sorry for the length. Do you mind if I ask how long or how experienced you are with psychedelics in general?

Be cognizant of your set and setting. Little things can have a huge influence on potentially life-changing stuff. You're afraid that you can't come back to the real world. Hey, now.

I think what you may be describing there is just a "bad trip." I don't like that phrase. I use "difficult experience." Then it can take on an educational angle which distances it from yourself as a sober individual versus your exposed ego. Then uncomfortable things can be learned from instead of resented. Since the sort of fear you're creating has to do with your own uncertainty it can probably be solved with a logical solution like reassurance.

I think someone in this thread already mentioned it, but I am a big fan of writing myself a note. Something reassuring- especially if the dose is new to me or no one is sitting. Maybe like:

metroid23,

You ate # grams of known Cubensies at 1pm because you wanted to. The effects will last a few hours and you do not appear sober. Remember to stay present and have fun <3 Everything will be OK. :)

One of the reasons I say to remember to stay present is because pretty much every negative experience I've had tripping can be traced back to some bullshit either in the past I was hung up on or in the future that I couldn't possibly conceive. I get reminded of it and then it's this endless loop of utterly pointless worry to the point of paranoia. But none of it resides in the moment where you should be utterly fascinated with life and amazed.

High Octane Nightmare Fuel: Psychedelic Horror Eschatology And What Comes After -kinds of questions can start with things like:

"Did I take too much?"

"Can they tell I'm high?"

"I think my heart is {beating fast|beating slow|beating irregularly|stopped|invisible|merging|transcendent}"

"This feels like a really long time or 'too long"

"When will I come down?"

"What if I can't come back to the real world?"

"What if I am changed? Will I ever feel 'sharp' again?"

"Am I loved? Did I make a mistake?"

If a sitter is around this might be when they would ask if you wanted to go play with crayons or pick a new song or try to read a book or anything other than whether or not you will come down again. You need distractions.

Weed is a great potentiator for pretty much every psychedelic that I've ever tried. It's useful for maintaing a peak or a little re-up towards the end of a trip. I usually refrain from smoking on the way up because I want to be sure my dosage is correct and not overshoot my expectations. Organics are imprecise. This also serves as useful reassurance that I'm going to come down eventually and I have a reasonable expectation of what the compound is going to do. If I follow those rules then pot is a friend. Be aware that Cannabis is a slight psychedelic in itself. In reality you are mixing those compounds and the effects are going to depend entirely on a whole cornucopia of factors.

As for the brain damage. I don't know. The leading cause of death is life, ya know. Are you doing to do psychedelics because the phone is ringing or are you ready to set the phone down with the answers you need? I've read enough that I believe the risk for long-term damage is small and that the current benefits of psychedelics outweigh those risks. If someone asked me to point to a journal and cite a specific article on how LSD is completely harmless, I couldn't do it though.

TL;DR: Write yourself a note before you take off, remember to stay present, as for brain damage to adlib someone famous- LSD creates psychotic behavior in those who haven't taken it.