"THE REAL ENERMY TO FIGHT TO REMAIN IN FAITH.
For we wrestle not against flesh and blood, but against principalities, and powers; against rulers of darkness, against spirituaspirituall wickedness in high places" - Ephesians 6:12
No soldier can ever fight and win a battle if he doesn't know the enemy he ought to fight. It's better to wage war against internal enemies so that it'll be easy to defeat external enemies. If you must fight the good fight of faith, the first inward enemy to fight to remain in faith are not limited to WORRY.
Worry is simply nursing a thought of potential problems that causes one to be anxious over a particular situation.
Jesus said, "Take no thought", that is do not worry. He knew that worry would prevent you from seeking first God's kingdom and righteousness.
It's not easy to live a worry free life this is why you must strive to kick it out of your life. The devil is the masterminder of this thought. Many prayers are worry-induced, this is why God isn't answering them.
You must also fight DISOBEDIENCE. You can't go far with God if you disobey His word. Disobedience is caused by both external (people) and internal (thoughts) forces.
The last enemy you must fight is the LOVE OF THE WORLD. How you see the world determines your pursuit about it. Many lust at the things they see, some are slaves of their desire while others boasts about what they have which is of the world (1 John 2: 15-17).
In Conclusion
These enemies are rooted in the thoughts of a man. This is why the writer of the Book of Proverbs said "Guard your heart with all diligence".