Don't blame all your problems and troubles on the Devil. Our number one problem is ourselves! The spirits we have the most trouble with are not evil spirits, but our own! The evil spirits are subject unto us. (Luk.10:19,20) It's our own spirit that is often not submitted to the Lord as it should be. Our own spirits are the ones that haven't yet been conquered!
If we're saved, the Lord will never leave nor forsake us (Heb.13:5), but we can lose His power and anointing for service by our disobedience. Our spirits have free will, and must be brought voluntarily under the Lord's control, or they can certainly lead us astray just as bad as an evil spirit! In fact, they BECOME evil spirits in disobedience! Our own spirits become evil when we disobey and are out of fellowship with God and out of His Spirit. (Rom.6:16)
God leaves the choice up to US which way we want to go, but if we'll make the RIGHT choice to go GOD'S way, then He can step in to help us overcome our weaknesses. We just need to yield to the Holy Spirit.--Then the Lord can rule our spirits if we'll just stay close to Him.