No, no, no, you aren't dumb for cancelling your lesson! In fact, I give you a thumbs up for thinking of the well-being of your horse. As long as you can distinguish between the line of "I don't enjoy the heat" and "The heat could potentially injure my horse and/or me" then you should be alright. Just remember that even if you can't have a lesson, if it is not TOO hot (you don't want to risk anything for your horse or yourself) even a simple ride just walking or some groundwork training or bonding lessons can be beneficial. It's super hot here too. The other day I did a walk/trot ride (mostly walking actually) for probably 30 or so minutes. I don't ride everyday either. I hope the heat is alright for youuuu and I hope it works out (: Try not to wish the summer away, it'll be winter soon enough!