There are two things that cause self-doubt in your life... 

1. Comparing your abilities to others - When you compare your abilities to others, you will begin to doubt your abilities. Instead of looking to God, you start looking at everybody else. You think these other people have more talent, more abilities, more education, or more experience. The moment you start comparing yourself to them, you're going to be filled with self-doubt. The Bible says, "It is dumb [stupid] to compare yourself to other people." (2 Corinthians 10:12) God doesn't want you comparing yourself to anybody else. He created you to be unique. He wants you to be you. Remembering your past failures.

2. Remembering your past failures is like putting a movie in your mind of all your mistakes and playing it over and over. It tortures you to death and it causes you to doubt your abilities. God does not want you focusing on your past. If you stay focused on your past mistakes, you can't get on with your present. And God can't use you in the future. God says forget the past. Don't let your past doubts, don't let your past guilt, keep you from being used by God. God essentially says, "I don't care what your past is. I want to use you!"

Scripture Verse

Oh, don’t worry; we wouldn’t dare say that we are as wonderful as these other men who tell you how important they are! But they are only comparing themselves with each other, using themselves as the standard of measurement. How ignorant! 2 Corinthians 10:12 NLT