If you feel fulfilled, you feel happy in a deep, steady way — because your life feels interesting, and you are doing things that seem useful or important. It is not the happy of a party, a snack, or a full stomach. You can have money and still not feel fulfilled. You can do quiet work and feel very fulfilled. People use it about life, a job, or a role: she feels fulfilled as a nurse. You do not use it about food. Though actually, fulfilled is also the past of the verb fulfill, which means “to do what you promised” or “to complete something”: they fulfilled the order. That is a different, very common use of the same spelling. American English writes fulfill; British English often writes fulfil. Both have fulfilled in the past. The “happy with your life” sense is the one here.