Antonyms for the phrase 'established oneself' could include words like unaccomplished, unknown, unproven, unsettled, or unstable. These are all words that imply a sense of uncertainty or instability, which is the opposite of what it means to 'establish oneself'. The phrase refers to an individual who has built a solid reputation or career path, and is generally well-respected and trusted in their field. By contrast, someone who is unaccomplished or unknown has not yet gained that level of respect or trust, and may still be working towards establishing themselves. Similarly, words like unstable or unsettled suggest that an individual has not yet found their footing, and may still be struggling to establish a stable, secure foundation for their career or personal life.