If you have a recurring problem, and you want to get rid of it, identify it. Make it as not-vague as possible. You might encounter something for the first time when you hear a recording where you are flat and dull-sounding, and think “Yuck. I do not want to sound like that.” Make a good description of it. Name it as clearly as you can. Beware of trying to name it by its remedy just yet. Keep the options open for now and just describe it.
Here is an example: I hear myself on a recording, and notes from C#4 to F#4 sound flat. I could jump to “My soft palate is too low.” or “I didn’t raise my soft palate.” But before we even get to that diagnosis or remedy, we need to fully describe what we are hearing. Is the sound flat in pitch, or in tone quality? Did the vowel change in an undesired way? Does that note sound or feel more effortful? How does it compare to notes that do not have that issue? What is its context? Ask a lot of questions!
After you have described it in detail, you can make a short name for it if you want to. If you want to call your issue “Thing 1” or “Agnes” or “Mr. Ugh” for short, that’s OK, but make sure you know all of Mr. Ugh’s characteristics when you encounter him. Or you can just call it “flat, effortful, and dull”. Whatever you call it, it helps to go beyond a one-word description like “bad” or “flat” or “yuck”.
When your teacher identifies a problem more than once, make sure you understand and get verbal feedback about whether this is Problem X or Problem Y. There are multiple muscular interferences that can cause flatness, for example. The more you can classify them, the better your association will be between how you got there and how to fix it. You may generally go flat for just one reason, or you may have many issues causing it. When you become an excellent describer of what you are feeling and hearing, your teacher can help you more. Often your feedback about an identified issue can help your teacher to explore new options to help you.
Learn what your issues are, what they sound like, and what they feel like. Then you are in a position to overcome them. Hear them, feel them, stalk them, investigate them, and then you can quash them. Yes, you will need to be brave.