There are a few questions that are unanswered regarding eternity in the new Jerusalem such as, who are the nations living on the earth and why are the leaves of the tree of life necessary for healing since everyone becomes immortal and glorified at the time of the resurrection and gathering?
Because they had access to the Tree of Life.. Once they ate of the tree of knowledge of good and evil they lost access to the Tree of Life and thus they no longer had access to the tree that caused them to be immortal...
Therefore this tree in the New Jerusalem is the same as the Tree of Life ( It may very well be the exact same tree ) and we shall have access to it so that we shall live forever more..