You need to have a game plan organized well in advance, before you even tee off on the very first hole.
This can best be done during your early morning cup of coffee and then during the ride to the golf course. No need to rush things, just relax and prepare yourself mentally.
Start on the first hole and in your mind put yourself through a number of scenarios. What happens if I pull the ball to the left? What if I fade into into the trees? Suppose I'm right in front of the bunker and the flag is tucked tightly right in front?
Now proceed to the following hole. Should I chip the ball low and let it bounce up to the green or can I better hit the ball high and hard so that it lands right next to the pin? What happens if I misjudge the distance and fly over the green
Before you realize it you've visualized all eighteen holes and you are feeling more confident. Hopefully it won't be raining when you get there.