in imperative form , you must remove ب . nafaset negah bedar is understandable but not correct. you have forgotten to add را , which make your sentence very formal , and it's only used in books. bedar , is also literary . but if you wanna speak like a normal farsi speaker , you must say , nafaseto negah dar .
نفست را نگه دار
نفستو نگه دار