oh, wait... wrong Bible. Let me get the right one....
Sorry, but when I trusted Christ, I was sealed until the day of the redemption of the purchased possession, not 'until I choose to walk away or until God no longer wants me'. God will not break the seal, I cannot break the seal. If either of us broke the seal, then Ephesians 1:13-14 is a lie. And, if Ephesians 1:13-14 is a lie, then how much more of God's Word is also a lie?
I made reference to Jeremiah 20:7-9 a few pages back, pointing out the fact that though Jeremiah decided he would just stop believing that God existed, he realized that was an impossible task. The Word was in him and shut up like fire in his bones.
It is the same with the sealed of God today. We have the Comforter, the Holy Ghost, who teaches us all things, testifies of Christ, and guides us into all truth. The saved cannot walk away, for they are guided by the Holy Ghost. The saved cannot stop believing, for the Spirit in us teaches us and testifies of Christ.
We are His eternally once we place our trust in Him as our propitiation for sin and for our Salvation.