Hi Kismet,
have you read this article by Erin?
The Medium and the Message
It helps you to distinguish between your message (your purpose) and its medium (how to express it). It sounds like you're trying to look at both of these at once. But I would recommend you focus on your message/purpose before you think about how you're going to express it.
My guides always tell me that purpose is a mission of being, rather than doing. It tends to be a set of qualities or attributes that are important to you and that you're already expressing. There is no need to look for it outside of you, because it's in you.
Just a few common purpose qualities I see in my practice:
Compassion
Courage
Leadership
Truthfulness/honesty
They can be expressed in an activity. For example:
Courage becomes defending others, getting others to stand up for themselves
Compassion becomes seeing the inherent worth in other people and showing it to them
Truthfulness and honesty are a desire to highlight the uncomfortable truths in the world and hold others to the truth
We are all here to bring different energetic qualities. Look at what is most fundamental to you as a person. It tends to be something you like to express in your life and share with others. You're probably already doing it.
Then those qualities can be expressed through a medium of your choice, like writing, teaching, assisting others, counselling, creating, inventing, etc