A few suggestions:
Be thankful that you don't already have everything you want. If you did, what would there be to look forward to?
Be thankful when you don't know something, this gives you the opportunity to learn.
Be thankful for the difficult times. During those times you grow and gain the ability to appreciate the good times.
Be thankful for your limitations, because they give you opportunities for improvement.
Be thankful for each new challenge, because it will build strength and character.
Be thankful for your mistakes. They will teach you valuable lessons.
Be thankful when you're tired and weary, because it means you've made a difference.