Is it permissible not to fulfill a vow one has made to oneself?

A person's promise to himself is nothing more than a determination, an intention, or a thought, and none of these things are binding. What is binding upon a Muslim is that which is an act of worship, and which is articulated with a formula that indicates commitment, such as: "It is incumbent upon me to do such-and-such," or "I vow to do such-and-such." This then becomes a vow that must be fulfilled. Therefore, there is no sin in not fulfilling what you promised yourself.