To be fair, "physician, heal thyself" isn't quite analogous to "put your own oxygen mask on first before assisting others." Their denotative meanings are very similar, but the connotation of "physician, heal thyself" is more along the lines of "get your own shit handled before you think you can go telling others what they should do," or, more broadly, "mind your own business." It's not supportive advice about the importance of self-care.