The heat is definitely a killer. I blew up twice this week thanks in large part, if not solely, to the heat. Keep sticking with it and things will get better as it cools off. It was actually better, though not quite cool, this morning and I managed to take advantage to have a pretty good track workout.
Remember (something I have to tell myself more often) you can run a race when you're not ready for a win or a PR. It's OK to run a race when you know the resuts may not be all that great. In fact, a few of those races can be useful in preparing us for the races where we do expect more from ourselves.