There are so many things flying at us at any given moment and we are used to juggling multiple things at once as we go through our day.
In your practice sessions, if you really want to fix an issue, pick ONE THING to focus on at a time.
Pick one thing. And then focus on that thing until it's as good as it's going to be today.
Let's say you picked tone quality.
Start with a note in the phrase you're working on and make it as beautiful as it can be. You might have to angle the airstream down. Or up. Or to the left or right. Or speed it up. Or slow it down. Or change the vowel shape you're holding in your mouth to move the focus of the air forward or back. You might move your bottom lip forward. Or back. You might try changing the direction of pressure of the lip plate on your lip - what happens if you press into your chin? Up towards your bottom lip? Ease off on the pressure? Raise your soft palate? Lower it? Change the amount of support you're using?
As you experiment, listen for where THAT ONE NOTE speaks and resonates best.
When you are ROCKIN' your B (or A, or E) and are able to consistently recreate the Most Beautiful Tone, move from that note to the next in the phrase. Or back to the one before. Is that note as beautiful? Start again!
Choose one element to really dig into in each practice session, and take notes so you remember what worked when you come back to it tomorrow.
Good luck and happy practising!