You'll have so much more fun in the kitchen and you'll make better food if you trust in your ability to play with your food. Cooking is best when you cook with love: love for whomever you intend to feed, love for your own creative abilities, and love for the ingredients. To this end, recognize that you will not always have every ingredient the recipe calls for; you just won't. Either you thought you picked it up, but forgot to, or you couldn't find it, or you didn't like the look of it, or it was too damn expensive. In any event, you'll find yourself without shallots, or with cauliflower instead of broccoli, or pork instead of chicken.
Don't fret. You can adapt. Trust your instincts, check the cooking time of the ingredient you're substituting, add a little less of something with a stronger flavor or more of something more delicate. Play. And sometimes you'll fail utterly, but more often than not you'll create food that pleasantly surprises you. It may not be exactly what you aimed for, but it may be gloriously unexpected.