What skydnsr said.
I am several times your age but I am a fainter and halso have a form of Ehlers-Danlos, and based on my very long experience, what you describe is the basic "faint." When you reflect on it, it seems like near-death, I know, but it is really a temporary alteration of consciousness due primarily to a drop in blood pressure. Your POTS condition complicates things slightly, but no, you were not close to death.
However, when the conscious mind gets around to analyzing a strange event like this, the whole thing can become rather traumatic, and unless dealt with, it can become sort of obsessive.
My advice to you: Ask for a referral to a counselor to discuss and de-fuse the experience.
But you really should ask your doctor if it's possible to die from a POTS attack, because if not, then you can be relieved of the worry. Again, this is just my limited knowledge that I'm going out on a limb on, but I think the risk of death from Ehlers-Danlos is from valve disease, aneursyms, and dissections -- not from POTS attacks. If you find out differently, please post the information, so I won't keep on saying something that isn't true.
You can be checked for valve disease and aneurysms, and when people have a dissection, there oftentimes was a pre-existing aneursym that was a red flag. So the fatal stuff doesn't necessarily come out of the blue. Since you have the diagnosis of Ehlers-Danlos, I assume you're being followed by an expert doctor. If not, you'll probably want to arrange a consultation with an expert, as soon as you can, so that your concerns about fatality can be addressed and so you can make sure you're getting optimal management.
To worry about dying from an unpredictable attack of something is a bad worry for a young person to have to live with. I hope you can get some solid medical information that will help you.
Take this for what it's worth from a nonphysician and nonexpert in Ehlers-Danlos/POTS, but it doesn't sound to me like you came close to dying. If you had actually stopped breathing, you would have been intubated in the ER, and you would have been admitted to intensive care. Probably the reason the nurses were keeping a close eye on you is because they wanted to make sure you did NOT stop breathing. Perhaps your breathing slowed down enough that the doctor got ready to intubate if necessary. But if you had actually stopped breathing, you would have definitely known about it later. You would have been told by the doctors and nurses. Respiratory arrest is a severe crisis. You wouldn't be guessing about it, especially after you've reviewed your medical records.
POTS attacks, if frequent, can be very debilitating, but each attack is self-correcting. When you stand up and your blood pressure drops, the body puts you back down in a supine or prone position by making you faint. Not pleasant, but with enough time in a horizontal position, your blood pressure and consciousness will return to normal. In the particular incident that you describe, it took about an hour for everything to re-regulate back to baseline.
The high heart rate is the body's way of trying to compensate for low blood pressure. It keeps enough circulation going that all the organ systems continue to work. The high heart rate also self-corrects when you get horizontal, as the blood pressure returns to normal. A person who is otherwise healthy can tolerate a heart rate of 187 or even higher, for some period of time. I wouldn't venture to guess how long, but for a good while.
That's my way of looking at this, anyway. Since your post had been up for a couple of days and no one else had responded, I thought I'd give it a shot. Maybe someone who knows more will post now. Oftentimes, it seems like someone who has specific knowledge won't post until they get a chance to correct the first person who responds, hahahah. Sorry you are having to deal with this. Good luck.