I also don't tolerate heat well and we went last August. I was so worried, but it actually wasn't as bad as I thought it would be. Yes, it was hot, but it was manageable. Here's what we did that helped:
Get out early! This was key. Then we went back to the resort from after lunch until about 4 before heading to the parks for the evening.
Get Hydro Flasks! You can get them on
Amazon. Fill it with ice and water at the resort in the morning and it will literally stay cold all day! You can get free cups of ice water at any quick service, so keep refilling it throughout the day.
Get mini fans. I know it may sound dumb, but it helped... a lot. I didn't like the misting type, but some do. We used ours when waiting in line for rides.
Plan on going to shows during the hottest times you'll be in the parks. We left for the early afternoons, so went to shows when it stated getting hot late morning and early evening when we got back to the parks.
I worried also, but it was fine. Just be prepared and have a plan. One of the days we were there it was 101! We were at animal kingdom that day and took frequent "breaks" by going into the shops to cool off. We had a great time and I'd totally do it again!