You're not doing anything wrong, the hot weather is your enemy.
A couple of options..
work slightly wetter, not massively or you will screw up your mix ratio and have service breakdown.
Work quicker, I'm not being rude or flippant, but try working faster, its easy to used to the speed of the product in normal circumstances and faff a little. If you know you have less time you can be more precise with your brush.
Cool your monomer, no don't put it in the fridge, but set your dappen dish in a small dish of cold water. Some techs are freezing their dappen dish in a shallow container of water over night ready for the next day.
You will also find your monomer will evaporate quicker and you'll use more.
In readiness for the winter, you will have the opposite problem and the nails will take longer to cure and your monomer may crystallise.