I've had two fear-inducing experiences, both pretty minor in my so-far short diving career.
1. Doing my first night dive, third in the day and pretty deep. I'm pretty sure nitrogen narcosis was involved, since my memory is still pretty cloudy. But all I know is that I lost my $1000 buck sealife camera on the dive and started to look for it. This was during AOW, so I had to let the instructors know before doing so, but still had one of them around me.
I noticed that the slight panic de-narc'd me a little, but the extra breathing it caused made me have to surface early. The good thing in my mind was that I recognized the symptoms of the stress and was constantly reminding myself to stop, breathe slowly and deeply, then continue. I feel it's important to be aware of your body and how the situation you are in is affecting it.
In the end, I found my camera on the next day, it actually floated right up to the boat. What a miracle, eh? I still won't forget how I felt the night before and try to mitigate the effects of nitrogen narcosis and panic by paying attention to how my body reacts to them.
2. Diving on the west side of Oahu with Ocean Concepts, we hit Keaau Corners and I couldn't equalize at all. Told the group to go on, since I was pretty close to the boat. Surfaced, managed to clear the block, and went down again. It was a little freaky diving solo, but as soon as I got close to the bottom I saw one of the instructors working with her OW student, so I hung around them until my group came back.
The big thing for this dive was my slurpy equalizations I was having, and due to the remaining block I had to constantly equalize. Kinda cut some of the fun out of the dive, but it was still successful. No reverse blocks or anything afterwards, either. The crazy thing is I didn't have any problems on the dive before that. But oh well.
Bottom Line: Pay attention to how you are reacting in the situation your find yourself. If you have the time, stop and think about it for a minute. If there's any way you can correct your reactions, do so. If you feel you can't, don't be afraid to thumb the dive. While I haven't had to do so yet, I hope that if I ever need to, I will. It's not about being brave or macho, it's about realizing that you can always dive again, unless you screw yourself up.
Peace,
Greg