How do you know what your purpose is? That's a big question. The answer begins with an equally big gateway. You begin by knowing how you know anything.
How does your brain work with your mind to produce that mental sensation of knowing something? You might never have thought about it. I know is both a concept and a feeling. You know you have the right answer when you get the desired result and, often before that, when you have a feeling that the answer is right. Knowing is more than thinking: it is thinking with your whole self - brain and body and mind.
This is how you know your purpose. It's not a matter of putting it into a slogan or motto. Those have their uses, but they are never a complete statement of what you want to be and to do. Purpose is the big container that lets you know whether a goal is right for you or just a distraction. Purpose is the path you are walking, even when it takes strange turns and you're not sure where it is leading. Purpose is the feeling that you are doing the right thing, especially when that right thing is a hard thing to do or when other people are not so sure you need to do that thing at all.
If I move your attention just a little to one side, if I ask, "Have you ever done something on purpose?" you will answer "yes" before you have time to wonder whether or not you really believe in having a purpose. A purpose isn't something to believe. It's something to intuit as the cause of the tangible results you see or hear or feel in your life. As you allow yourself to notice that you have sometimes done things "on purpose" you can become curious about what more you can learn about this desired future that contains all the effects you will have on the world.