It is impossible for the Devil to defeat you unless you give in to him! "Greater is He that is in you"--Jesus--"than he that is in the World!" (1John 4:4) The Devil cannot win! He's already been defeated by Jesus! (1John 3:8b) If you keep fighting, the Devil cannot win! He can only win if you surrender, if you give up and run and wave the white flag of surrender. But he can never win as long as you keep fighting!
The Bible says, "Resist the Devil, and he will flee from you." (Jam.4:7) As long as you keep resisting, he'll keep fleeing, but if you stop resisting, he uses his lies and temptations to try to persuade you to quit, that you have a hopeless cause, so you might as well give up and quit and surrender and leave the field to him, "It's no use!" he says.
So don't listen to him! Get tough and fight back with the "Sword of the Spirit, which is the Word of God!" (Eph.6:17) Don't let the Devil say "Boo!" and scare you out of God's Will! Stand up and fight, and he will flee from you!
Put your faith in the Lord and His Word and you're bound to win! Put on the whole armour of God, take the Sword of His Truth and attack the Enemy! (Eph.6:10-18) The victory is already ours! Jesus is the Victor over all!