I know what you mean. A horn blowing when I'm riding sometimes causes my adrenaline to spike (not in a good way), even though, most of the time, it has nothing to do with me. When I'm being hyper-aware, which is often necessary to safe riding, any little incident can easily cause me to overreact.
I haven't found a perfect solution yet. I try to give myself a little talk before starting out on a ride, for example: pay attention/don't zone out; you're out to enjoy yourself, not fight with morons; it's not all about you, others need to use the road, might be having a bad day; etc. But sometimes I'm in a rush to get going and I forget, or I get caught up in the moment anyway.
Just one more thing to be aware of (ha ha), I guess, when cycling on the road.