As you play tournament golf, the game is just different. You goals of why you play the game change and the reasons you play the game change. All of this creates more pressure on yourself during your round. As you are playing tournament golf there are techniques that you can use in order to help you accept the situation.
Monitor your walking pace - again, this is very effective and can be done at anytime. Most of us do not care about going to the mall however, you might earn some bonus points with the spouse and you would be able to monitor what is a slow, comfortable or fast pace for you. Try walking at different paces during the day and monitor how your body feels at the different rates.
Monitor your breathing - this is very effective and can be done at anytime. We all have started breathing faster (whether we knew it or not) on the first tee, on the ninth green before a big putt or on eighteen to win a match. It is useful to catch yourself at home while you are relaxed and monitor your breathing in such a state so you have a baseline.
Ensure humor and fun are part of your round - this is sometimes harder to do than others. Even if sometimes you have to laugh to yourself, I have always found it possible to do though. At the club level, if you are not having fun in team events, then you need to find a new partner. I have found that most people want to enjoy themselves even at the "higher" levels of the game.
Focus on your pre-shot routine - once you have a routine ingrained then this is a good way to click into auto-pilot. It is critical that your routine is repetitive and does not waver. A change could cause anxiety and mis-trust in your golf swing.
Whistling or singing - Fuzzy Zoeller is infamous for his whistling while he is on the golf course. I tend to do this myself on the course from time to time to blow some steam. I have tried not to sing as it may result in a two stroke penalty if anyone else heard me sing! It may work for you though.
Picture a calm place - sounds corny to some but it works. My in-laws live out in the country in Kentucky. While we are there, I walk everyday and there is a big barn that sits on tops of a hill in the distance. These walks are very relaxing. The Kentucky quarter happens to have a picture on the back of the coin that reminds me of that barn I look at on my walk. When I mark my ball with it, I am able to remember my relaxing walks and I can feel my body relax. This place might be the beach for you, your back yard or a sunset. The point is it does not really matter as long as it relaxes you.