When you learn a new technique or a different way to play or sing something,
it's like your brain has planted a little seed; the idea has birthed
a tiny little plant of a better way inside of you. It's very small
and fragile, because it's just been learned - careful, you might forget
it on the way home from your lesson! You've got to nurture that better,
newer way until it becomes a solid plant you can rely on, one that stands
on its' own and carries you along. An additional idea won't help
that little seed grow, that's just another idea, another plant to grow.
What makes this new technique grow (a technique is a physical ability, whether vocal or instrumental) is practice, physically doing the better thing again and again and again until it becomes as normal as the old way is now (not you thinking about it more). Every time you work on the new way, the plant gets a little bigger, a little stronger.
Unfortunately, right beside the new, better way is a great, big, mean, ugly plant you want to get rid of - THE OLD WAY! The wrong way, the slow way, the sloppy way, the traditional way that has hurt you and held you back, but it has one great advantage over the new awesome way you're trying to learn, over the tiny plant you're trying to nurture to useful fruition.
The advantage the old way has is.........it's a habit. That great big plant is the way you've always done it, the way your body feels is the right way, the 'natural' way' (because you've done that bad way forever). That's why it's so darn huge, you've been watering it with practice for years, maybe decades (lots of the ways we play or sing things are from habits picked up in life, not just how we make music). The second you stop watering that new way, the old way rushes in and says, 'I'll take over. Surely I feel better. This is so comfortable now, the old, familiar way, isn't it?'.
Imagine that big ole method or technique leaning over the baby brand new way, trying to intimidate it and replace it. To you the old way says 'Surely you don't need this new way, surely you can do just fine with me, lots of people use me, you don't need a new way, we're all quite happy just the way we are, thank you!'. So, you've got to keep watering that better way, that better idea, keep paying attention that you are doing it the way it's supposed to be done until its strong enough and natural enough to take you to the next place in your musicianship - AND you can't find that old, bad habit anywhere at all because your new, better way has become the natural, normal, comfortable way you always use.