Are you sure of yourself? Are you confident that you can do what you set your mind on? Do you honestly believe that?
Let's take a lesson at what God says about doubt.
Why do you doubt? What is it that starts those thoughts in your mind? Is it because you see someone else doing better? Or is it because you don't think you'll ever get to your own point of satisfaction?
Proverbs 23:7 says "For as he thinks within himself, so he is. He says to you, "Eat and drink!" But his heart is not with you."
Your thoughts are a big factor in your life. You can't give energy or power to negative thoughts. Yes, your life is going to have trials and tribulations but you can't dwell in your situation. In those moments, find a reason to go on. Think of your future, when things get better, when you wake up and receive that good news. It all starts with your thoughts.
1 Corinthians 2:11 says "For what person knows the thoughts and motives of a man except the man's spirit within him? So also no one knows the thoughts of God except the Spirit of God."
God knows your desires. He knows your wants and needs before you even open your mouth to ask. God sees how hard you are working, He knows what your goals are, He knows what path to guide you through, and He knows exactly how to do it.
Doubt stems from a place of frustration, loneliness, stress, and anxiety. God is with you, He hears you, He's walking with you daily, and He's here for you no matter the day or time. Put your trust in Him and He will show you all that He's capable of.
If you'd like to read more about Doubt, please see Job 42:2, Psalms 56:5, and Proverbs 15:26