You may have heard the saying "Practice makes perfect." In my own experience, that is not accurate. In my experience it would be more accurate to say "Practice makes permanent." If you put in the right kind of practice, all is well and good. If you have put in the wrong kind of practice, you are worse off than before, because you have just carved your faults in stone.
I find this whenever I take up my intermittent singing lessons. My teacher is still, after years, working on the basics with me. I am trying to make sounds that isolate certain aspects of my voice. Our lessons go like this:
Him: Sing Ah!
Him. No. Not 'Ah!' Ah!
Him. No. Still not right. Ah!
Him: Closer. Like this: Ah!
Him: No. Wrong again.
This goes on for much of the lesson until, finally:
Him: There! That's it. Now do it again. And again. And again.
At the end of the lesson.
Him: I think you've finally got that nailed down. Can you remember how it felt to sing that?
Him: Good. Go home and practice that from now until the next time.
Later, at home.
Me: What was that again? Oh yeah. Ah! not quite. Ah! Better. Ah! Got it. Ah! Ah! Ah! Ah! etc.
Him: Did you work on it.
Me: Yes! And how.
Him: Good. Let's pick up where we left off. Ah!
Him: What the heck was that?
Me: That was Ah! Just like we worked on last time.
Him: No it wasn't. Back to the beginning. Ah!
Him: That's not it either.
.If you really want to know what it is like to bang your head against a brick wall without actually banging your head against a brick wall, take up singing.