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Relax. Breathe. Dive. Remember that this is a hobby that you want to have fun with. Reading too much interwebs chat without any relevant experience will just make you anxious.
Find some local divers, perhaps through your local dive shop or even on here, and go do some easy diving with them. This could also be accomplished by signing up for an Advanced Open Water Class (it is really not "Advanced", just slightly more advanced than you are right now) and maybe even the Rescue Diver course.
As you start doing some diving and learning from those you dive and hang out with, the majority of the issues you bring up will sort themselves out. Just have some fun and use common sense in the "is this risk acceptable for me?" department. Not sure you can handle the current? Sit one out. Friends planning a dive that seems a little too deep for your current experience? Sit one out.
Please find some good buddies and some easy dives and have some fun!
theskull
Now I have signed up for a nitrox course. I know professionally a bit about physiology, and it just makes a lot of sense to me to use nitrox in many situations. And reading through the PADI manual, it basically tells me to just rely on my computer. Now that really starts to make me nervous - somehow I prefer to have at least a good understanding of what the limits are, so that in the unlikely event that my computer tells me nonsense, I can check for plausibility and, if warranted, substitute my own more conservative judgement. Am I just too skittish as a new diver might well be, or are there corners cut?