Calm, Cool & Collected
Golf can be a very frustrating game. When you play this game, you compete against the course, wind, rain, heat, bugs, course casualties, unfortunate breaks, your opponent, and the biggest adversary, yourself.
A lot of factors can affect the outcome of your round, however, controlling yourself is the key to more consistent positive outcomes. If you can go into each round accepting the fact that you will have bad breaks, poor shots, wind issues, etc., you will be more prepared to keep your cool. It is acceptance of what golf throws at you. It builds a base that helps manage your thought process. Just accept it, be calm, and let "it roll off your back".
This is the struggle, it takes practice and focus to do this if it is not in your personality to do so. However, working on this will bring great things.
I equate this demeanor to Ty Webb in Caddyshack. He didn't let anything get to him. That attitude is a great quality. Frustration, anger and doubt only intensifies a less desirable outcome. Remember, thinking negatively attracts negativity, so remove it by staying calm, cool, and collected and you'll see positive results.