I think most people (read: adults) have a hard time when they try a new skill. Most of their discomfort is due to a lack of understanding of that stages of learning that you go through when an adult student. For reference I'll list what I consider the 4 main stages of adult learning:
1: Unconscious Unawareness: You don't know what you don't know. Example: Diving looks amazing on TV / movies and is something you really want to do. Looks so effortless and fun.
2: Conscious unawareness: You find out that there are alot of aspects of the new activity that you really don't know how to do. This is the toughest stage for adults as we do not like "not knowing". Note that children typically do not have this problem as they are not expected to be 'good at things' right off the bat (generally speaking).
3: Conscious awareness: With practice, when you focus and think the new activity through, you can perform it. IMO, this is where you'd want to be before taking any exams (theoretical or practical). For instance, you'd want to be at a point where your scuba skills learned in the pool are 'repeatable' before you attempt any ocean dives.
4: Unconcious awareness: You perform skills without even thinking about it. When asked what you just did and why, you'd probably have to think about what it was and why, because you are so adept & used to performing it that thinking about it would be a waste of time.
Now, even though I've been a professional instructor (NOT anything to do with scuba!) since 1998 and have trained adults from 18 - 50+ years of age, I had a hard time with my first pool session. And I knew the above and realized why I was getting frustrated.
However, having taking some one-on-one instruction has gotten me to the point (over the last 3 classes) where I am basically caught up with my group and I am looking forward to getting into the ocean.
Hopefully you will be able to keep going and not get frustrated. People have different comfort levels with being under water / getting water in their mask / not having a mask on underwater etc. If you appreciate the fact that your learning timline may not be what is designed into the course and take the pressure off yourself by saying " I will learn at my own pace an realize that some skills I'll have to practice to become adept at" you will enjoy the experience and really enjoy the feeling of accomplishment that you get from performing a skill that was originally tough for you.
Ok, that got longer than I expected.
Good luck out there and most importantly, have fun!
Bjorn