For me, fulfillment isn't any one particular thing, it is the total of my life, my experiences, and the relationships I have. Within the relationship I have with my Master, knowing that I enrich His life, enriches mine.
For me...fulfillment was very elusive- until I figured out who I was, and started living in a way that no longer denied the real me. Once I dropped the pretenses, stopped struggling to be things that I was never intended to be (and never wanted to be), and embraced those things that were of importance to me- fulfillment simply became a very welcome byproduct of every day life.