Why is it so hard to disguise or unlearn an accent?
As an intellectual problem, it seems pretty easy. For example, you might coach a New Yorker to simply pronounce his r's-"New York," not "New Yawk"-but moving from theory to practice is often difficult.
How come?
The answer has to do with the way we learn language in the first place. Learning a language is a lot more than learning a bunch of pronunciation rules. It's also a matter of training the speech muscles of your mouth, throat, and lungs.
After we're six years old, it becomes harder to learn new ways of making sound. The way our speech muscles produce vowels and consonants become somewhat fixed, as do our overall speech patterns of intonation and rate.
Changing your accent isn't an intellectual problem at all, it's a matter of learning a whole new set of muscle controls.To expect someone to quickly learn a new accent would be like expecting them to quickly master a perfect backhand swing if they've never played tennis before. Explaining the rules simply won't do it. It takes determination, patience, and a lot of practice.