The phrase "am beside oneself" is often used to describe a state of extreme emotion or agitation. The antonyms for this phrase typically reflect the opposite of these emotions, such as calm, collected, composed or serene. These words can be used to convey a sense of peace and tranquility, whereas "beside oneself" has a sense of chaos and turmoil. Using antonyms for "am beside oneself" can help to create a sense of contrast and balance in your writing, and can be particularly effective when describing contrasting emotions or situations.