That’s a good question, Andre. I don’t know of any accent reduction coaches on Italki, but we`ll see what others have to say.
Phil might be able to help you with it.
You write pretty well. Why are you concerned about the accent? (By the way, if you say “ya`ll” you will sound like you are from Atlanta, or New Orleans. So you really want to sound like a Southerner? We don`t say that in California.
If you can't find a teacher/coach, you could record yourself on this website and see if native speakers here are willing to provide feedback:
You could also take a look at the website of the U.S. embassy in Brasilia for exchange programs that would get you to the U.S.
Finally, check out meetup.com for your city. There may be Americans interested in face-to-face conversation exchange.
If you really want to acquire a particular accent, as in your case an American accent, what you really need is a linguist who is trained in the pronuncation of American English (any of several varieties, based on region) and it would help too if the person knew the pronunciation of your native language. These people are specially trained in speech therapy, and are sometimes called speech therapists.
I can`t speak for other languages, but elementary schools in the US always employ speech therapists to help children (native speakers of English) pronounce sounds they are having trouble with. A substantial minority of children need special help with this. I really don’t know why this is, but most of the children eventually grow out of their mispronunciation problems with help.
You need to know how to reproduce certain sounds in your target language and be taught how they are made in your mouth and vocal cords, from a physiological perspective. Foreign language teachers don’t have training in this usually. The “listen and repeat after me” approach will never work, because all the student does is try to match the new sounds he or she is hearing with familiar sounds in his or her native language. And although there may be some close approximations you can make in a few certain cases, almost no sounds totally match up perfectly from one language to another. Hence, you get an accent. That’s where the accent comes from.
The voice coach, Gareth Jameson, is teaching stage accents, not native pronuncation. He covers a few salient aspects of an accent, just enough to convince an audience that a character is Russian/German/French/American. To a British audience, the American R (and a few vowel changes) makes a British actor sound American. Of course, an American would instantly note that the actor's accent was American-ish and not actually their own regional accent and not a standard broadcast media accent.
Here is a youtube video of an Indian company teaching an American-ish accent to call center workers. There appear to be about one hundred lessons. I have no opinion on the quality of the course.
https://www.youtube.com/watch?v=1C3rKpAIc14
@SHL
There is published research on age and ability to learn the phonology of a second language. In general, before puberty, children pick up native phonology and grammar. After puberty, with sufficient exposure, children pick up native grammar but not native phonology.
@Uncle Sam
There are multiple American accents. Your regional accent is one in which the cot-caught merger has not occurred.
https://en.wikipedia.org/wiki/Cot–caught_merger