Mental game matters in ALL sports, but the nature of individual sports, such as golf, poses an added challenge because, at the end of the day, you can only rely on yourself (and you only have yourself to blame).
Repetitive practice, in particular with coaching, continues to build confidence, which will benefit your mental game. You must ALWAYS take practice swings from behind the ball. This keeps the ruminating away from the actual hit so you can get a sense of what you are aiming to do. By separating your practice swing from the actual hit you make space for thinking about the hit in practice which frees up your mind to feel the right hit when it’s game time.
If you tend to do your practice swings over the ball then you are forced to realign yourself; which takes time, and changes what you are learning. By taking your practice swings from behind the ball you are building confidence in your aim and alignment by providing a wider lens. Also, being behind the ball allows you to scan the grass in front of you identifying possible hazards to avoid. Many golfers benefit from using something like a leaf to place in front of the ball to help with aim. These tricks allow you to pick up environmental cues that allow you to feel the lie of the ball.
Remember, first tee jitters are normal! But, by never skipping a practice swing and always taking your practice swings from behind the ball you are building the muscle memory required to be confident in your swing. If you are experiencing first tee jitters, try to dial in on what you know you can do. Trust your training. Give yourself confidence by narrowing in on one task or goal. This tight focus will allow you to leave the rumination for practice. For more detailed golf coaching by Trillium Rose, check out https://www.youtube.com/watch?v=fhSLPqAR0PY&t=1725s