Consider that perhaps you don't find your "calling." Instead, it finds you.
The only "trick" here is that you have to be reachable by putting yourself out there, and keeping in mind that you're fishing, not hunting. Coming back empty-handed is not a sign of failure, it's one round in a process of elimination.
Volunteer work is a good start, missionary work is even better. Or pick up a holiday shift in retail or a temp job. Help out at homeless outreach, animal control and domestic violence centers. Attend a service at a church, a temple and a mosque. Go on a ride-along with the local cops. Visit your national parks and memorials.
Most of this is an exercise in forced association with people and situations you'd otherwise overlook-- you'll hang out with the poor, the wealthy, the criminal, the pious, and everything in-between enough to at least get a sense for the breadth of the struggles of the human condition-- and present you with a buffet of experiences broad enough to encounter a perspective that resonates with you.
My pet theory is that triumph over struggle is the real driver of happiness and the only universal purpose any of us have. Once people transcend the struggle, there is nothing left for them to overcome, and they never again feel any sense of satisfaction.
ESID, and not everyone is meant for servitude, but it works for me. Best of luck to you.