I was talking with another group about having less energy to practice in the summer. I was surprised to learn that others don’t feel this. It was suggested that I may have a balance or self regulation problem. I wonder, what has me so out of it in the summer?
That would be a very good Tarot question! 
Summer (always depending on your location of course) has a very outgoing, extrovert energy. Especiall before the summer solstice the energies of the year are rising - this calls many people to go out more, be in nature or meet people, be more social etc. So, we are connected more to the outside world.
When fall and the autumn equinox comes closer the energies slowly subside again. We´re tuning in to cooler weather, to staying at home more and to connection to the inside world and this makes it easier to connect to your magic and spirituality.
For some people, like me, summer depression is a real thing and I experience symptoms of fatigue and lack of motivation.
So, it can have many reasons…
I’ve never heard of it, but if people can be affected by not having enough light in the Winter, I don’t see why you couldn’t be affected by too much light in the Summer.
I think you are onto something here. With the prolonged heat of a Summer it affects your whole body in so many ways, lack of sleep due to the heat loss of important electrolytes, lack of appetite thereby nutrition deficiency, sun rising earlier, noice from wildlife. Over a very short period of time this brings extreme fatigue, energy levels can diminish quickly as your body is trying to protect itself by using the energy it has keeping your vitals stable. You might find that you get more cravings such as ice cream one to cool yourself, fats to replenish energy to try to be more motivated.
As @Amethyst has said in winter there is a well known condition called SAD so yes I do believe there is one for the Summer months and the severity can affect people differently.
Hopefully, now that the weather is cooling down and the mornings and nights drawing in you will notice a difference.
Blessings 
People who are in the autism spectrum (like my daughter) find summer very difficult because of the hard, bright sunlight, more people being outside and the city just being very loud (buzzing with tourist here in Vienna), sweating and clothes sticking to your body can become an unbearable sensory overload.
Ew, that does sound awful!