I think ones purpose is usually found at the intersection off all your interests. Try writing down general areas you are interested in. Nothing specific. For example mine might be Racing/vehicles, travel, tech, Games ,Innovation, Psychology. Then you think of a way to combine all(or most) of your interests and bring it to the world.
I really like Mastery by Robert Greene. He gives a lot of examples of people discovering what they are called to do and brought it to the world. What got me is a lot of masters had no hidden talent at what they were called to do.
Generally speaking it's to maximize my life satisfaction both short-term and long-term. Specifically what that means may change over time. I have a trial and error approach to it, I'll try a direction and measure it against my current situation. I can't know in advance what's the right path for me, but I know for damn sure whether it's right or wrong for me once I go down a path.
I don't fear making a few bad choices, it's better than not making any choices.
"Purpose" isn't about what you can do for you as much as it is about what you contribute to society and the legacy you leave behind. Lifting weights, eating right, making lots of money, are more lifestyle goals. Purpose is the umbrella under which everything you do fits.
A man needs Long Term Perspective. Every decision you make: job, health, spending, friends, gf/LTR/wife should all be consistent with where you want to go long-term. That's why it'so vitally important to choose a LTR or wife who shares your long-term goals and vision.
I started a business back in 2001. It's my purpose. I'm a gifted public speaker and executive coach. So my purpose in life is to assist others in reaching their true and full potential, mainly in business and their career. I've authored a couple books and have a tv spot so even if I kick the bucket tomorrow, the words I wrote and my spoken word will carry on.