I just returned from a week on GC this past Sunday. I dove the North and West sides of the island. As a little background, I had just finished up my OWD cert the week before we left. So this was my first experience in the ocean and I had never been deeper than 45'.
Needless to say, when we arrived at the first dive site and the DM explained that we'd be doing a 100' wall dive, my anxiety was rising steadily. I reminded Jerry (the DM) of my previous dive experience and he assured me that I'd be just fine and to just 'stick with him'. I knew that I just needed to get in the water and to the top of the reef (~50' down from the boat) and I'd hopefully feel better.
As it turns out, I couldn't have been more right! I made it to the top of the reef, ears equalized, everything in check, and I started to get into my groove. As we approached the edge of the wall my excitement/anxiety had pretty much equaled. When we went over the edge and I looked down at the blue abyss we were suspended above and the giant wall we were flanked by and I knew I was experiencing something truly amazing. We continued along the wall for about 10-12 mins (at ~100') and then back up the wall and on top of the reef for the remainder of the dive. It was a great first experience in the ocean, I was hooked.
Assuming you have a good dive op and DM, they should be able to pretty accurately judge your skill level just by talking with you. And then their thoughts will be confirmed after they see how you behave below the surface. My confidence in the dive op was founded on the recommendation by the LDS that I certified with and it was confirmed from the minute I stepped on their boat. Jerry must have felt confident in his assessment, otherwise he would have directed me to stay on top of the wall with the other DMs group. That first dive was exactly what I needed to build my confidence for the rest of the dives. By the end of the week, I was the first one jumping off the boat and down to the reef waiting for the other divers. It was awesome to see my confidence and skill level grow over just a week's time.
I guess my point is, certainly don't push yourself beyond your comfort zone. But don't allow your initial anxiety at the surface to dictate your whole dive. You never know how you'll feel when you get down there. If at any point during the dive you feel like you're pushing your comfort level then, by all means, ascend and get back to where you are comfortable. Your DM should talk with you about possible scenarios before the dive, so you'll have options when you get down there.