There are a number of synonyms for the phrase "overextended oneself," which generally refers to a situation in which someone has taken on too much responsibility or committed themselves too strongly to a particular task or project. Some possible alternatives to this phrase might include "overcommitted," "overreached," "overexerted," or "overstretched." Other synonyms could include "overworked," "overburdened," or "overloaded," all of which suggest that someone has taken on more than they can handle. Whether you're talking about a professional or personal situation, using one of these synonyms can help you to convey the same basic idea as "overextended oneself" in a more precise or nuanced way.