I thought of two more things that I'd like to say to people preparing to go on a mission:
Ask people. Ask returned missionaries, both recent and old, current missionaries, and ward mission leaders/missionaries. Ask them if they think that their mission is a sacrifice. Ask them what they wish they had done before they left on their mission. Ask them what they wish they had known. Ask them for advice. Ask them what the most surprising thing is/was about their mission.
Pray for the Lord to tell you what He thinks of you. This can be a life changing prayer. That realization of what the Lord feels about you and your efforts can fill a hole of longing and inadequacy that you don't know you have. You will feel the pure love of God.