Okay, so first, the structure.
The road to purpose goes like this:
> First, you have an interest
> > Then you partake in a lot of activity
> > > After that, you *might* have a purpose.
Career and self-help writers often mislead people, I think, because the predominant message is:
"When you have a passion, you'll just know!"
No you won't. There is not a single thing in my life which I currently enjoy that I "just knew" would be the best thing ever. All of them were fun at first, but sucked almost immediately after. Some of them I kept doing anyway.
Purpose is built, not found. Henry Ford, Steve Jobs, Tony Robbins — all were obsessed with a vision, yes, but only because their actions led them there. They weren't born with it. Not even Steve Jobs. Jobs didn't know he'd be building computers his whole life. All he wanted to do was "stand at the intersection between technology and humanity." With that wide of a goal, he could have had any number of careers.
Reminder — you can be more than one thing. Life is long. I keep thinking of Richard Branson, who started a record company then got interested in cell phones so he did that instead then got interested in space travel so now he's doing that also.
Here is a suggestion.
The next time you get an impulse, follow it. Don't just think about it, take some kind of action on it. If you want to write James Bond novels, awesome. Write something every day. Do that until it is not fun at all (which shouldn't take long). Here's the key — now do it some more.
Fulfillment is often after fun and beyond pain. The goal is to find an activity that gives you pleasure even when it is not necessarily enjoyable.
When you get there, you might be on to something.