The antonyms for 'art beside oneself' are composed, collected, tranquil, and serene. When someone is composed, they are in control of their emotions and able to act rationally. A collected person is also calm and poised, while someone who is tranquil is free from agitation or disturbance. Serenity refers to a peaceful state of mind or environment. These antonyms reflect the opposite of being 'beside oneself', which suggests a loss of control or extreme emotional reaction. It is important to aim for a state of tranquility and composure to avoid feeling overwhelmed or out of control, allowing us to approach situations calmly and rationally.