The phrase "turned corner" typically means that something has improved or changed for the better. Antonyms for this phrase include "took a step back," "turned back," or "went in the wrong direction." Alternatively, "stalled out," "came to a halt," or "plateaued" could also be used. These phrases indicate that progress has slowed or ceased altogether. Another antonym could be "missed the opportunity," which suggests that a chance for improvement was not taken advantage of. In any case, it's important to keep moving forward and to not get stuck in a negative state of mind.