One thing that helps -- if you're in the right head space -- is to make a mantra out of your performance goal... In this case your breathing... Choose a word that resonates with you and then every time you notice yourself "holding your breath," think or say the word and do the "action." In the case of breath holding, maybe the word exhale...
"Who cares if your "deadly art" was originally practiced in a temple in some obscure corner of Bangladesh if an ill-tempered girl scout with 6 months of boxing can knock the hell out of its practitioners?" --Mike Driscoll