954 David, I hear your frustration. When I was doing more in-kennel projects I hated mid-summer because it limited how long I could train a dog on field trips during the work day. Mother nature will always be one up on us there.
I would try to do as much as I can indoors, in the shade, early morning and evening. I would also do frequent shorter sessions that needed to be done outside and would make sure to get the dogs back to a cool place between sessions.
With a focused lesson a lot can be accomplished in a short amount of time, but if the dog goes into the session already panting it will still be difficult so have the dog fresh.
In New York, I LOVE fall and spring for dog training. Winter usually bothers me more than the dogs when training outside. Your dog probably loves winter!