I spent the early part of my adult life riding around the ocean on a submarine that was two weeks older than me. It was there that I came to understand the principle of redundancy - as the US subs had all kinds of extra devices and systems to gaurd against the possibility of a single failure keeping you from getting to the surface, or keeping the reactor from melting through the bottom of the hull, or to make electricity, water or oxygen, etc.
In my musical life - I can't stand to go to a gig thinking that failure of a single piece of gear is going to torpedo the entire event. I bring spares for everything - even one complete backup PA unit.
When something does go wrong - all that i am then faced with is the awkward fifteen minutes to get the spare installed. That serves me well on the stress minimization front.
I try and carry enough spare bits and pieces to cover every possibility. I get more stressed if anything goes wrong during the setup, once the gig starts I just have to carry on. It helps if I'm fronting out the band since I can make the odd joke or two with the audience before dealing with the problem.