A song learned badly can take longer to unlearn than it took to memorize.

Practice makes permanent before it makes perfect. A mistake repeated with confidence lays down the same kind of track as a correct version, so the correction then has to compete with a path already worn in. That is why fast early learning can cost you later. Undoing is slower work than doing.

When you are learning something new, go slowly enough to be right rather than quickly enough to sound fluent. Check the fingering, the pronunciation, the grip early, while it is still cheap to fix. And if you already carry an old error, be patient with how long replacing it takes.