For many amateur golfers, it is a common issue to have high hopes for a great round before you go out to play. You think positive and build yourself up before you hit the tee. Heck, you might be so "ready," you clean your clubs the night before your round, get the new Pro-V's ready, etc. you get overly prepared thinking you are doing the right thing, with determination and focus. You think that this is going to be the best round yet, or the big win.
This is where positive thinking can have a negative impact because you get yourself all jacked up when you do this. You have become emotional about this round before it has even started. It creates additional unnecessary pressure that will throw off your thinking, nerves, and tempo. It can result in the exact opposite of what you are looking to achieve.
If this is a problem for you, try going into each round with minimal prep. Just have fun and enjoy the day. Keep your emotion low. Tell yourself to not focus on score or results, just focus on each shot. Then let go, and don't give a shit about anything else. Go have fun, be carefree and don't let anything get to you. Stay focused on your shots, but eliminate focus from your results.
When you release the unnecessary pressure you put on yourself, you remove a weight on your back that usually drags you down during your round. It is a choking point that many of us amateurs are unaware. So just relax, don't give a shit and have fun :)