I just run into the opposite problem. My sim psyched himself up at the mirror to become confident and acquired the Vain Street quirk in the process. After a few hours the tense+2 from not admiring himself enough appeared, but I do not have the Admire Fine self interaction at mirrors anymore. So unless I clear off the quirk or buy Never Tense (both if which I do not want, because that sim really is vain), the tenseness is permanent?
(Although now that I'm writing this, was there maybe particular mirror needed? Hanging, standing? I tried a wall-mounted one in the bathroom only, maybe I should have tried others.)