You've already gotten all the quality advice there is to give, lol, not much to add- Personally, I live in AZ, and it gets to the 120's in the summer routinely. I usually either have to ride in the heat or not at all, so the best option is pretty much to stock up on water, get some good sunscreen, find a nice hat/ helmet with viser, and get used to it... In AZ, it's not much cooler in the morning/evening, so that's not the best option.
Fat is also an insulator, so try kicking up your work-outs/ starting to work out to get more muscle and less fat... I've packed on a lot of fat lately, with the stress of Junior year, and I can DEFINITELY feel the difference now that summer is here. But frankly, I think that's the best option- don't avoid the heat, get used to it/ condition yourself and your horse, try to keep less fat on your body, WATER AND SUNSCREEN, of course, lol, and I like to ride in the morning so that I can jump into the pool afterwards
. My horses also get sprayed down/ bathed frequently and after every work out. One of my horses is black, and chubby, so I have to be very careful with him in this heat- he needs to lose weight like I do, LOL.