Have a "fatigue" swing available. I used to have that problem before and it was shankitis around that time or hitting fat.

Try not keeping a written score. Focus more on the shot execution and not the result.

Eat something and do not forget to drink lots of water. That is water! not sodas and gator aid.

try not to think about what you could shoot, just take one shot at a time. You will be taking money from your brother in no time:)

Could be your getting tired which could cause swing changes.

Read that book. One of my favorites. I would also suggest "Zen Golf". Also, what is your nutrition like? I trained for and ran my one and only marathon back in 2004. Learned a lot about nutrition that I have applied to my golf game. In the last 7 years my hcp has dropped from 5-6 range to 2-3. I'm 49 now and playing the best golf of my life. Nutrition is a big part of it. So is flexibility. For nutrition on the golf course, I only drink water, except maybe a Gatorade/Powerade in super hot conditions. For food, I eat 2 ZonePerfect bars, one on the front 9, one on the back. Last summer they came out with a new "sweet & salty" line that doesn't get all gooey in the heat, as so many of those protein bars do. They're available all over. Target has the best prices. No candy bars or soda on the course for me. Good nutrition = steady blood sugar level = steady brain function. Good luck!