Spirit Confusion

This morning I was reading in 1 Samuel. In Chapter 16 David had just been chosen as God's anointed, and in verse 13 it says the spirit of the Lord came mightily upon David from that day forward. Then in verse 14 it says the Spirit of the Lord departed from Saul, and an evil spirit from the Lord terrorized him. This is where I got really confused. Is it only in the Old Testament that the spirit could depart from us?? I thought the Word said that the Spirit, once in us, could not depart from us. I looked up that verse next and it reads As for me, this is my covenant with them," says the Lord. "My Spirit, who is on you, and my words that I have put in your mouth will not depart from your mouth, or from the mouths of your children, or from the mouths of their descendants from this time on and forever," says the Lord. Isaiah 59:21 Even though this is still in the Old Testament, I know that Isaiah is a prophet, and is speaking about the Messiah in this verse, so does that mean that before that time the Spirit was able to depart from us but now that Christ left us with the Spirit now it can not depart from us?? The Spirit was with God before He even created the earth from what I read in Genesis 1:2 Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters.
The more I ponder it the more confused I get, so for right now maybe I should pray on it for a bit. I thought that writing it out would help me grasp it a little better, but so far it is only bringing on more questions! I sure hope this is not a theological debate question, because the last time I got myself wrapped up in one of those I had a headache for a week!