Yesterday I was mentioned in a comment from @twizzle 's story. Thanks, @lucjah hopefully, I can make some things clear in this post.
I can only talk about my point of view. I'm not driving an ambulance vehicle for work, but I think with about 1200 hours a year I'm not far off a professional paramedic (they are doing about 2300 hours per year) here in Austria.
I'm taking some sentences from @twizzle 's story, I hope that's okay for you. :)
"I wonder if most paramedics are constantly stressed and near to quitting."
We are not constantly stressed and near quitting, as I've told in another story of mine (Invisible Scars), we have some techniques to release our stress. I know a professional who's in the job for 15 years now.
Also, over time you learn how to not let things come near you. Most of the patients I work with are unknown to me, even thou my family and friends are in "my catchment area". (Fun side fact, I've driven the pregnant girlfriend of a coworker to the hospital so she could give birth to their child).
Another thing you learn quickly is to shut down after you "walk out the door". So you're not taking the stress home with you. Some do workouts, some work at other companies or simply have a family the spend time with. Everybody does what works best for them.
"They all look very calm and in control when I see them but under the surface are they crumbling away through too much stress day after day after day."
Well, we have to stay calm, because if we can't stay calm, how could we expect that from patients?
I'm not calm in every situation, but, at least here in Austria, we are practicing situations and disease patterns so often, when you're coming to such a situation, some kind of "paramedic-instinct" kicks in.
I don't know how I should describe this, but when you're in this "instinct"-mode you're not thinking how some of our equipment works, you're basically just working with it. Every handle is in place automatically.
The realization comes later when you have time to think about the situation. Don't get me wrong, situations can get overwhelming but somehow the massive amount of training lets you stay "clam".
I don't know how it works, but it works and for me, that's the only thing that matters because it helps me to get a good job done. No matter what happened.