Jehovah's Witnesses talk about living "forever" and even place God in our own experience of time. This is not the same for other religious people, especially for those who are members of non-Witness Christianity.
For them, God transcends time on a plane known as "eternity." On this plane, God is, was, and will be forever from our point of view, but in reality, it was from this plane that, according to religionists God "created the timeline."
Imagine drawing a line in the sand. You can see its beginning, its middle, and its end all at once. In this same way, it is taught, God sees the beginning, present, and future. God doesn't predict the future. God sees what those with freewill do with it. God can therefore tell the future to humans, reporting it from God's vantage point.
Forever isn't an eternity. You got the two mixed up. Let me explain.
Forever is the constant flow of time. Time is the constant change of things and the measurement of that change.
Eternity is not time. Eternity is where God is, says theology. Time is that "line in the sand." Forever would be a line that "went on forever," something that would only be if God drew such a thing, so to speak. Eternity would always, from this standpoint, transcend time. There is no change or time in eternity, there is just "is."
The Jehovah's Witness concept of forever is flawed in that the Witnesses never look close enough at what is eternal and what is the change of time. The Witnesses teach that God is a timekeeper when in reality God is not under the same constraints of time. They often speak of God living forever and God is eternal in the same breath when, in reality, God is eternal, not "living forever."
You are right that "eternity does not exist for mortals," but it does exist for immortals. That is why the apostle Paul said that mortality had to be swallowed up by immortality for death to be done away with.--1 Corinthians 15:53.