You have your own voice, because you are God's unique creation made in His image both outside and inside.
But as FishCatcherJim pointed out, man was designed to have a spiritual overlord and it will be either Jesus or Satan. We have to choose which thoughts we take. Just having an evil thought is not a sin, for temptation is temptation, not sin, it's when you meditate that thought and begin to entertain and agree with it that it will become sin.
But no, Satan cannot read your mind, but he can give you thoughts. Again as Jim pointed out, the devil watches us and he's been doing this stuff for 1,000s of years, he's pretty good at it. He gives thoughts in the first person so that it seems like it's you, when it's really not.
His name, the Devil, in Greek is actually a compound name that means "one who strikes repeatedly" as with a rock or a ball. He is named that because mental warfare is how he operates, to pressure you with repeated thoughts until you eventually agree with them and speak and act those thoughts. That is what spiritual warfare is all about. He has to get your agreement, because he has no authority whatsoever - he needs you to do it. So, through suggestion to both your mind and flesh is how he operates.
Ask the Holy Spirit to sensitize you to Him and to help spot any thought or pressure that is not of God and then take the thoughts captive, bind whatever spirit it trying to make you depressed/stressed/fearful/nervous/etc.