Stop fertilizing your flowers. Give your flowers some relief from the heat by letting them rest a bit. Fertilizing during the hottest days of summer won't revive them, it will encourage new growth that will require more water. Give them a break until mid to late August.
However you can replace tiring annuals by reseeding heat lovers like: cleome, cosmos, marigolds and zinnias. While you're at it, start some fresh herbs from seed too. Basil, borage, coriander, oregano, sage and thyme will all sprout quickly in heat and linger to fall.