One of the most naked indicators of an inexperienced cook is a sauce that breaks, thins, or tastes flat. You have to manage sauces: heat, time, consistency, flavor. Often, young cooks will think that recipes are the solution, but sauces are more technique than ratio. You need to develop a sense of what is going on in a pan: how butter sputters, how raw flour tastes, how liquid reduces.
One of the most common errors is to hurry the base. In a basic flour-and-butter roux, a lack of patience results in insufficiently toasted flour that tastes raw, no matter what else you add. The fix here is sensory, not time-based: over medium heat, stop toasting the flour when it smells nutty and is a shade darker, then slowly pour in your liquid. If you start to see lumps, you are probably adding liquid too fast or your mixture is too hot. If you reduce the heat and whisk well, you can avoid lumps from fully forming.
Another novice mistake is adding sauces and seasoning towards the end. Remember, seasoning is a cumulative process. Adding salt towards the end and tasting still yields a dull result, even if it’s actually salty. Add a little towards the beginning, a little more towards the middle, and a little more at the end. They dissolve in different ways and affect the whole dish. Over time, you get a sense of how flavors mature rather than just increasing.
If you can spare fifteen minutes a day and have a pan, some butter, flour, and milk or stock on hand, practice, practice, practice. Make a small quantity of sauce, watch for color, smell, consistency, how it clings to the back of a spoon. The first day, take the sauce off the heat a little too late. The next day, take it off too early. The two sauces together will teach you more than an ideal result done just once. If you overthicken and the sauce breaks, whisk in a tablespoon or two of warm milk or stock; see how quickly the sauce smooths out. This eliminates the terror of “ruining” the sauce.
And when you get stuck, simplify. Using cheese, herbs or spices masks your technique and can obscure issues. Master the basics before introducing those variables. And as the pan begins to lose its mystery and starts to feel like a utensil, you’ll find that even recipes you’ve never seen before become less intimidating, because the fundamental technique of manipulating temperature and motion is innately understood.