Each one of us has the capability to make a difference in the world. Some people devote their life fighting for a cause, eg eradicate poverty. They've found their purpose.
Other people work as comedians and on top of making a living, they're happy they are able to cheer people up as life is stressful as it is. Some may do voluntary work, cleaning up their neighbourhood or teaching children with special needs. They are helping to make the society better.
Since we live in a society, there are certain norms and we are expected to do certain things. It doesn't mean we have to do them. If you don't want to get married, then you're going to be unhappy if you do. Do things you truly want from the bottom of your heart, what makes you happy.
Try to help others along the way. You don't need to devote your whole life to a good cause. You can still have your own life, live it the way you want as long as you're not hurting anyone. You have a notion of what's good and what's bad, stay away from the bad and you'll be doing only good.
When you can help someone in need, don't hesitate. That's the beauty of goodness.
If your purpose is still not clear, maybe you need to clear your mind a bit. There may be too many things going on in your head, your vision may be impaired. Try to meditate. Stop the thoughts and listen to your heart. See what makes you happy and how you can spread that happiness to others. There, you'll find your purpose.