wow, i've never heard so many tree guys admit fear - how cool is that. my advice to brand new climbers is if you're not afraid of heights you're stupid. it would be like not being afraid of the ocean, right? it can kill you. simple. respect and awareness of that is the key. good for you guys, pitching in support.
my 2 cents - when i first started working for myself i made a rookie mistake (i wasn't a rookie, though) that almost killed me. i tipped a 10 ft top into a high tension wire and got zapped. climbing for the year following that, or maybe more, was a jaw-clenching experience. partly it was because my adrenal system was fried and i would get totally random shots of adrenalin at weird times. the other part was what some of you guys are talking about - suddenly being more aware of how "squishable" i was. being stubborn (and also because i love love love climbing trees) i just plowed through it. eventually i got most of my "guts" back, it just took a while. but even now, there are times where i get sketched out - hazard tree removals, wires too close, lots of wind, etc.
my 2 cents comes from 14 yrs of martial arts training. breathe. if you stop and pay attention to your breath, it will help you feel more grounded. feel your lungs expand, feel the heaviness of your arms and legs. almost guaranteed to calm you down if you give it a minute or two. and when you're not like thrashing anymore, you get rational.and then, like someone else said, think about what is worrying you so much and find a rational way to put that worry to rest. works for me. hope that helps somebody sometime..
it's dangerous work. it makes sense to be worried, but don't let it run you - use it. doublecheck knots/biners. re-think rigging scenarios. etc.
long post. sorry bout that.
k.