I empathise with all of your original post OP - I was almost exactly the same, although it wasn't so much fear of pain/passing out as having the runs whilst at work or whilst out. Or, having the runs at all actually. I adopted very similar coping mechanisms to those you describe.
I improved with 10 weeks of CBT when I was quite young - it helps you to understand that the thoughts are not rational, and generally speaking your body has a way of not letting you down at crucial moments! I've made it through to 50 and my worst fears HAVE happened, but not with the frequency and heinous consequences that you might imagine. CBT teaches you to try playing out your worst fear and saying out loud what the absolute worst thing to happen would be - when you get it out there, it sounds a bit silly and you realise that as an adult, you'll handle it. People with medical conditions have to do it every day, I always remind myself. I do have IBS although not too badly.
Sadly one thing did happen which set me back slightly - DH and I bought a campervan, and on one of our first trips away I got up in the night with mild tummy discomfort for a poo, in a terribly cramped, hot and fusty camper bathroom which stunk of chemicals - came over peculiar and fainted, scaring the crap out of poor DH. The fainting was followed by an hour of the worst diarrhoea of my life, in the same cramped and boiling hot chemical toilet cupboard, although after that I was absolutely fine. So random...
Now whenever we go away I am nervous of it being too warm in the van, nervous of eating too much and feeling full, and nervous of having to poo in there especially at night. Which means I've reverted to some of my old coping skills - eating very little when we're away, eating beige and safe "low residue" foods like white bread and biscuits and rejecting any fibre etc. In my worst times I used to pop immodium like sweets, but I don't do that anymore!
My record for not going is 9 days. Terrible really! It's a shame because my DH suffers with anxious tension at home, and going away in the van is the only time he is able to fully relax - and conversely, now when we're away I get really anxious!
Probably no help but you certainly are not alone - I know it feels like you must be crazy and you could never tell anyone in real life. Thank God for the internet, didn't have that when I was a teenager and going to several chemists in town to score my poop pills...
Oh - I kept a diary/journal thing for a while, which was useful at tracking the wayward thoughts and getting them in perspective.