You can be "on a mission." Searching for something that meets a specific requirement, like the perfect dress for an upcoming occasion.
You can be part of a mission, serving a ministry or organization with clearly defined goals or services.
I happen to know several people who are incredibly busy. I often wonder if they ever sleep because they are involved in so many different things. Occasionally it makes me feel a little less-than. Like I ought to be doing more. "So-and-so is doing all these wonderful things and involved in so much! What am I doing?"
But I have to step back and realize that I'm not called to do everything. The mission that has been set before me and the things I am involved in are what I've been charged with for the time being. I want to do them the best that I can, and if I stretch myself too thin with other good things to do, none of them will be done well. And I'll be worn out and worthless. (Yep. Been there, done that!)
Way back when, we were all charged with an amazing mission. Jesus took these two old commands to remind his followers what we were meant for:
Love the Lord your God with all your heart, soul, mind and strength. And love your neighbor as yourself.
Even when we feel like we don't have a specific calling or purpose, this is the place to start and all else will fall into place.
And when you get a little overwhelmed or your scheduled overloaded, go back to basics. That's the ultimate mission!