Maybe some relaxation exercises while you ride? Before shows, my drama teacher would have everyone breathe in for four counts and out for four counts, four times. It helps to relax and remember to breathe!
Also, try sitting in the saddle, and bunch your shoulders to your ears, tense your back, thighs, your entire body. Then just let yourself DROP! Go from that tensed state, to just letting everything fall down to proper posture, with your head high!
What really helped me to relax was when my instructor put me on a lungline and had me canter with my eyes closed and my arms out. It may sound odd, but just taking some time to realize that you can just be silly at times and not have to stress about making every little detail perfect yet.
But most importantly smile! Maybe even sing! Come up with games that you can do while warming up/cooling down yourself and your horse. That way when you go to work, you're starting with a blank canvas, and your warm up doesn't have to be perfect, and your cooldown can be a time to just relax in the saddle again!
Sorry this is so long, hope it helps!