I've read lots of tips on how to calm yourself when nerves or anxiety are running wild during a specific situation. They're typically meditative in some sense, such as:
--Breathing deeply and in a controlled manner. Let your lungs expand. Feel your stomach rise and fall - oftentimes we only breathe with our chest. Stomach breathing will generally be deeper and slower and will require more concentration, thus focusing your mind.
--Zeroing in (your vision) on little physical details in your surroundings. Perhaps the chair you're seated in has a pattern? Or you can count ceiling tiles. Read all the writing on the walls in the room, such as the signs in the doctor's office. Little, short things to give your mind a task without trying to blow off the thing it's worried about, which may just cause more anxiety if your subconscious feels you aren't prepared for the anxiety-inducing event. At least in my experience!
--Positive affirmations. Now this isn't "Think positive! Everything will be alright!" The concept here is that you repeat (out loud or in your head, whichever) the outcomes of the situation causing you anxiety as positives or neutrals to counteract the negatives that are flowing through you. For every "I'm going to mess up!" you must then say "I'm not going to do anything wrong."
--My therapist often asks me to explain my worst case scenario when discussing anxiety over an event. Often I can't answer - I can't, when thinking logically and putting my anxiety into conscious thought, come up with anything terrible! Realizing that the situation, no matter the outcome, isn't that big a deal can help. From there you can walk through plans for if something were to go wrong.
Focus is most important in my case. The second I let go the anxiety will, unfortunately, flood right back in. That's why I'm better at controlling my anxiety during/right before a specific situation than when I'm nervous about something days or weeks in advance - as in a time period too long to maintain constant calm.