I don't know how much you have thought about it, but eternity for us is not the same as eternity for our Lord. Everyone is going to live into eternity. Some will spend eternity in hell and those who know the Lord will spend eternity in heaven. And, when we think about living for eternity we mean we believe that we will live on and on.
Jesus' eternity, though is different. You see, we have a beginning. One day you and I were born to earthly parents. We celebrate our birthdays each year. I've celebrated over a half century of birthdays, now. You might say my eternity started whenever I was conceived. Others might say my eternity started when I was born. I might say my eternity started at the age of seven when I gave my heart to Jesus and was born spiritually. The point is, for human beings eternity has a beginning, but has no ending.
Jesus, though, has no beginning. He was with God in the beginning. That's what it says in John 1:2 --- He was in the beginning with God. (NASB)
I confess that I do not understand that. I can't comprehend a "no beginning" life. My mind tells me everything has to have some beginning, including Jesus. But my faith tells me that His Word is true because He is truth. So, when I read that He was with God in the beginning I just accept it. I accept it with the understanding that it makes His eternity and my eternity two different things entirely.