The difference between a purpose and a mission is the level of passion and vigour you bring to your endeavours.
When you are enthusiastic about something, you have a purpose. When you are passionate about something, you have a mission.
This extra boost in energy comes from your soul. You may have many purposes throughout your life, but usually you will only have one mission.
The search for your mission begins with your search for meaning. What is it that you came into this life to do? What is the gift that you were born to give?
When you know the answers to these questions, and act on them, you will begin to find not just meaning; you will begin to find fulfilment.
Sometimes it takes years or decades to discover your true purpose. Take your time, and be patient. Do what is in front of you and follow your inspiration. Eventually, it will all be revealed to you.