You need to be involved in something that you're truly passionate about. How many of us spend a large amount of time and energy doing activities that we don't really care about or are just going through the motions with? Truthfully, the majority of us. It's a scary fact, yet because these mundane activities and lifestyles may bring temporary satistfaction, a temporary fix, or money, we allow it. Try exuding your energy into something that you are truly passionate about for a while and take note of the sense of purpose and refreshment you will receive from it!
You may need to express yourself. Feeling unfulfilled can be an unexplainable feeling. You can't quite put your finger on why you feel this way, or what has caused it. In this instance, you may not be as in tune with your feelings as you have expected. Sort out your feelings through self expression. Tell a close friend how you're feeling, write out your thoughts, take a moment for self reflection, or have a conversation with yourself.
You could be holding on to dead weight. Nothing new can emerge if we are busy hoarding the old. When we feel unfulfilled, it isn't because we need that "old thing back", but because that old thing no longer serves us and our "new" is awaiting us, but has yet to be let in. With old stagnant energies lingering around, it has likely already fulfilled its purpose and is no longer useful. Make room for the new by dropping the dead weight. Dead weight could be old habits, thoughts, physical items, our situations with other people, or even our own attributes. You decide what it is for you.