How can I stay cool while golfing?

I am 13 and today I played 18 holes starting at like 8:30. I started really well until I got around the 6th hole. I started to double bogey every hole. I realized I had a headache and the sun was blazing down. I started to lose my focus and started to pick the wrong clubs for my shots ( 170 yard shot I would use a 5 iron but I picked a 6 iron) When we made the turn I got an ice water but I still had a terrible headache. I drank all the water after the first 1 1/2 holes on the back 9 but I still had a headache. On the 3rd hole I completely fell apart and I skipped that hole and while the rest of my group was putting I put ice on my face from what little ice was left from the ice water and I lost my headache right away so I could focus on my game and I played how I should of played the whole day for 2 1/2 holes (When my ice supply ran out) And I played pretty bad for my last 3 holes. Usually on the course I play on I shoot low to mid 80s and no higher than 88 but I shot 92 today. I am wondering how I can stay cool like I did for 2 1/2 holes today. I have a golf tournament on the 28th so I need some tips on how to stay cool (other than drinking water because I drink plenty of it) Throughout the day. Also would a hat help me stay cooled because I don't wear one but I can.