No problem! No gagging with the endoscopy- you'll be asleep/unaware by the time they put the scope in. Nausea isn't common with that type of anesthesia, the way it is with general anesthesia, but I always request it anyway because of my phobia. The endoscopy truly isn't too bad ime.
For colonoscopy prep, I would ask your doctor what your options are. For example, you might be able to take laxatives in pill form vs drinking a bunch of liquid laxative, which will probably be easier and more comfortable. When you know what kind of prep you'll have, definitely Google it for tips! If it's a liquid, people often suggest things like keeping it as cold as possible, and that was really helpful for me.
There's not a great way to make diarrhea a better experience, but I always make sure I have something like reading materials or a movie queued up in the bathroom to distract me. You can try to make your surroundings as comfortable as possible- candles, soft lighting, maybe a blanket/pillow nearby in case you can't go far but don't want to stay on the toilet the whole time. For me, the stomach cramps hurt at first, but after a bit it's just gross, lol. I also take my anxiety meds beforehand, and make sure that I keep hydrated as much as possible. And after the prep is all finished, I often do feel good for a few days, like my body has gotten rid of a bunch of stuff that might have been sitting around in my digestive system, since I'm often constipated. Not good enough to do it all the time, of course- but it can sort of help press the reset button, if that makes sense? So that's the silver lining. I can understand having fear around it, but it truly does feel different for me when the cause is laxatives and not food poisoning or something, because I know it's supposed to be happening.
I know this is a hard process to go through, especially with this particular phobia, but it's not as scary as it seems. Good luck- you got this ❤️