First off, don't feel bad. Things like this happen, and unfortunately, some unscrupulous suppliers encourage it.
Second, take heart. Venus Flytraps require living prey to stimulate their trigger hairs even after the trap has closed. When those hairs are not stimulated sufficiently enough to cause a seal to form, and digestive juices to start flowing, the traps simply open back up in a day or two.
now, if the petfood is high in fat, it may damage the plant, and you most likely will lose a few of the traps, but this is by no means the end of your plant, especially if it was healthy prior to your science experiment.
Here are some basic care instructions:
Never feed it anything but an insect (of course you know this now!
Never fertilize it.
Put it's pot in a saucer filled with Distilled, RO, or Rain water. Avoid tap if you don't know it's mineral content. (and don't confuse it with chlorine, setting water out for a day won't do the plant a lick of good.) You can buy distilled water in a gallon jug at a grocer like HEB or Albertson for about 48 cents a pop. it's pretty inexpensive and ecenomical when you only have a few plants.
Put your plant in a southern facing window sill (assuming you are in the norther hemisphere, if you are in the southern hemisphere, put it in a norther window.)
finally, if the traps have not closed all the way on the food, you might be able to get it out by gently opening the traps with something... maybe the end of a q-tip stripped of cotton or something...
if the worst does happen, and your plant does die, don't despair, try again. They are wonderful plants, and NOT hard to care for!