I live in Central Florida and I have lived near Atlanta. Central Florida during the summer is even worse.

Rule 1) Drink a lot of water. Top off before riding and drink while riding.

Rule 2) Cool clothes and sun screen.

Rule 3) Backing off the pace a tad may keep you feeling a little cooler.

Here is the rationale. When you get hot, the body diverts blood flow to the skin in an attempt to vent heat. I have found that riding at high altitude in Colorado (up to 12,000') is no more taxing and maybe less so then riding in hot, humid FL.

The extra blood flow to the skin means the muscles are getting less and maybe that is causing your sensation of "fatigue".