What my club golf championship taught me

1. Golf is hard. Competitive golf is harder still. Competitive golf is an endurance test  – a test of both, physical and mental endurance. Train for it as such. Make sure you are physically capable of taking 4-6 hours of standing, walking (and staying balanced) – in possibly extreme hot (or cold) conditions.   If you are NOT physically up to the task, one of the first things that will go – is your balance (during your swing). It is hard to stay balanced for the entire 18 holes – unless you have prepared your body (specifically your lower body) to handle it.  It is harder still to do this for a period of 3-4 consecutive days.
2. There is a difference between playing two sets of 9 holes (say 9 before lunch and 9 after) – and playing 18 holes straight without a break. Most amateurs (especially club golfers), tend to take breaks during an 18 hole round. Competitive golf does not permit such breaks. Which makes it that much more demanding physically.
3. More than ONE swing thought will ruin you. Just carrying a swing thought with you to the COMPETITION is not a bad thing in itself. PROS do it even (see the READING LIST below for a compilation of swing thoughts from the world’s greatest golfers). Amateurs tend to OVERDO it. They take A thought for their short game, another one for their long game, another one for bunker play etc. While all of these are OK during a regular, non competitive round, I guarantee you, in a competitive round, your brain will NOT do well with multiple swing thoughts. It will already have a LOT on its mind – without being burdened by multiple swing thoughts. Keep it simple – take ONE – and ONLY one thought – which can help with your SHORT and LONG game. It may be as simple as ‘Smooth on the backswing’ or ‘Smooth on the downswing transition’ or ‘Finish your follow through’ – or my favorite, Turn your back to the ball.
4. Golf can be unfair – you can hit all solid shots on a hole – and still make a triple bogey  (On a par 3, I hit a tee shot that landed on the green-  rolled off, got stuck close to a tree and gave me no shot – except a left handed punch out. I punched out left-handed – only to watch my ball fly into a hazard :). The drama continued. It took everything to just salvage a TRIPLE on that hole. Rather than curse your fate, it is helpful to realize that you are playing a sport that has an element of UNFAIRNESS built into it. The very RATIO of the playing surface (400 yards length – multiplied by at least 40 yards width – to the hole) to the target (a 4 inch hole) – makes it an inherently challenging endeavor (for a hilarious take on the EVOLUTION of golf, enjoy Robin Williams’ take on it).  
5. One bad hole can weigh on you a lot more than in a casual round. It is important to tune out bad holes. Again, mental toughness – and accepting the inherent unfairness built into the sport , can help you accept this. Also, keep in mind that EVERYONE will have a bad a hole or two – no matter how steady their game may seem (again due to COMPETITIVE golf being different from regular golf).
6. To play well, take the simplest possible swing – and the simplest possible swing thought. We all have a few different versions of our swing – some that emphasize our LONG game more , others that are more approach shot centric – and others that focus on a killer short game.
7. Lastly, this should be no revelation to anyone, bring your  A short game. All tournaments are won and lost around the green.  You can leave your driver at home and still win – as long as your short irons and chipping (and putting) are all holding up.  However, the converse is not true. If you forget your short game at home, no matter how well you drive it, you are going to suffer miserably during the competition.

Summary

Bobby Jones famously remarked:

There’s golf and there’s tournament golf, and the two bear little resemblance.

Anyone who has played competitive golf, in any form, can relate.

APPENDIX – Reading list for competitive golf tips (for a more general reading list, visit these golf instructional books recommendations)