Okay. This is what I tell all of my students and it seems to work. Two things.
First- As you are going to the dive, visualize all the things you will do. Start from unpacking your gear. Step by step go through equipment set up, safety checks, getting gear on, go through your entry, bouyancy check, descent, equalization, then your skills. As you do this, think what could go wrong, and how you handle it. You will know that you have an answer now to everything, so you are prepared for any small issue that might crop up- even in your head. This does help you focus and know that you are prepered and ready, and it will stop you from randomly fretting.
Second. You know the panic cycle. Recognize when you begin to feel anxious. I tell people to put their hand out like a stop position. Turn your palm to face you and as you inhale, draw your palm to to your reg. On the complet inhale, your palm is at your reg. On the exhale draw your palm out and away from your reg. On the complete exhale your arm is out straight. Now repeat this as you breathe. Your hand is moving in time with your breath, so it slows as your breathing slows. Focus and watch your hand. think "slowly in....and out....." This is really just focusing and a bit of meditation for breath control. It will calm you down and give you time to relax. Then you think about what is scaring you and realize that you can handle it.
It is normal to feel nervous before your first dive. Some nervousness makes you more alert. Just try to relax and know that you are ready, and it is easier in the open water than in a pool. Chlorine sucks. And you will things to look at! Boy is that awesome. You will be fine.