Being nervous has kept me out of the water on my last trip here because my sister in law didn't dive and I didn't trust a stranger to be my buddy at this point.I think other people have already said this, but being nervous isn't a bad thing in my opinion (unless, of course, it keeps you out of the water).
I've been diving for almost 15 years with a resurgence over the last 5 years as I decided to go down the tech/cave diving path. 5 years later, and a couple hundred dives (most of those in caves or overhead environments with deco), I still get a little nervous before almost every dive. But instead of allowing that feeling to overwhelm me, I try and use it to focus my thoughts and energy on dive planning and execution. And 95% of the time, once I submerge and "lock in" to the dive plan, that feeling goes away and my brainpower is dedicated to the dive itself and my team.
Being nervous is just your brain and body trying to keep you alive in the face of the unknown. And that's a good thing.
I didn't do a boat dive yesterday and I don't know if it was due to nerves or good sense. 30 knots wind, whitecaps and waves breaking out in the sea. I thought I would be over my head in skill level. I wouldn't have snorkeled in those conditions and I'm very confident snorkeling and swimming in the ocean. But I thought, "Don't make my last dive a bad one so I won't want to dive again." I'm not sure if I should have bailed or not. Ugh.