The phrase "in a dither" typically means to be nervous, agitated, or confused. Its antonyms, therefore, would be words that convey a sense of calm, composure, and clarity. Some good options might include terms like serene, tranquil, composed, at ease, poised, collected, and unflappable. These opposites of "in a dither" suggest a state of mind that is confident, relaxed, and in control. By contrast, the verb dither itself denotes a sense of hesitation or uncertainty, so antonyms for this usage might include words like decisive, resolute, confident, and unwavering. Together, these words provide a helpful contrast to the feeling of being "in a dither.