A single day. A moment. An act. An experience. Friends. Family. Career. Hobbies. Passions. Beliefs. Morals. Values. Your thoughts. Your feelings. Your emotions.
None of these, by themselves, wholly define you. You are, in fact, a culmination of the sum of these and much, much more; inevitably unique in nature. So do not distress whenever you fail yourself. Avoid expectations of perfection, and instead bask in your shortcomings and weaknesses, knowing that you are so much more than any one of them.
You define yourself, and definitions change all the time.