While walking on the water, the Apostle Peter became afraid and started to sink. "Lord save me," he cried.

Do you lose faith? Like Peter, we are quick to forget. We've seen God do miraculous things. He has stumped doctors. 

Trust in the Lord with all thine heart and lean not unto thine own understanding. In all thy ways acknowledge him, and he shall direct thy paths. Proverbs 3:5,6

Doubt comes when we can''t figure things out. Quit trying. Give it to Jesus.