Ashley Brace, from Ebbw Vale, Blaenau Gwent, could not compete at the Commonwealth Games because of her kickboxing past.
She said she was unhappy with the support from the Welsh Amateur Boxing Association (WABA).
Chairman David Francis said the organisation did all it could to help.
The Association of International Boxing Associations (AIBA) said Brace was ineligible to compete for Team Wales because she had competed in professional kickboxing in the past, which she denied.
Organisers of the kickboxing tournaments she competed in have also since confirmed she did so on an amateur basis.
A further rule states any boxer returning from another individual contact sport should apply to an international committee to become eligible to box again.
Brace said she was never told about the paperwork, which had to be submitted by WABA, and felt the association let her down.
She now trains at Bristol Boxing Gym and has her first professional fight in Newport on 30 October.
She added: "I would never go back to Team Wales after this, because if you don't have confidence in the people who are supposed to be fighting your corner, at the end of day, what's the point?"
Mr Francis said the whole incident was "regrettable".
A former Welsh amateur boxing champion said she is turning professional as she does not have confidence in the governing body.