Strong and powerful is he who [obediently] carries out His Word.

Strong and powerful is he who [obediently] carries out His Word.
“The LORD utters His voice before His army, For His camp is very great, Because strong and powerful is he who [obediently] carries out His Word… “Turn and come to Me with all your heart [in genuine repentance],
With fasting and weeping and mourning [until every barrier is removed and the broken fellowship is restored], Rip your heart to pieces [in sorrow and contrition] and not your garments.” Now return [in repentance] to the LORD your God, For He is gracious and compassionate, slow to anger, abounding in lovingkindness [faithful to His covenant with His people]…”
Joel 2:11-13 (AMP)
You remain strong and powerful when you obediently carry out to the letter what God instructs you to do.
God told Joshua to be strong and courageous, and he will continuously have success. 
(Joshua 1:9)
He did. 
However, when he looked at the appearance of men, he was deceived. (Joshua 9)
Through genuine repentance based on the Word of God, he became unconquerable again.
Have you missed out a step in your relationship with God?
Once you come back to God with a real turn around in your heart, He will restore.
Joshua identified where he missed it and turned back to the Word.
Don’t stay defeated and allowing feelings of being unsuccessful to weigh you down.
Don’t just go through the motions.
Don’t stop reading God’s Word and abandon fellowship. 
Turn to God entirely in your heart.
You will experience that indomitable strength again. 
HE is eagerly waiting for that unbroken fellowship with you.