I live in Washington, and we’ve been having about 90-100 degree weather, and my trainer always warns me about the horse’s internal body temperature. What I usually do: I don’t ride in about 90 or above. But if the supposed temperature is supposed to be about 90, I ride between 7am and 9am, so the temperature is lower. You didn’t mention what kind of work you do, or how hard you work, but when it’s really hot, I usually try not to work my horse too hard.
Another recommendation is that you could work on groundwork, showmanship, stretches, etc. or you could pamper your pony with a extra special grooming or bath.
Good luck on finding your answer, and have a good day!