When we think of the tree of life, our minds usually go to Genesis first. After Adam and Eve sinned by eating from the tree of the knowledge of good and evil (thereafter knowing evil as well as good), besides kicking them out of the garden, God had to put a guard around the tree of life. Genesis 3:24 AMP 2015 says cherubim were permanently stationed with a "flashing blade that turned round and round [in every direction] to protect and guard the way (entrance, access) to the tree of life." This was because, had Adam and Eve eaten of this tree after having sinned, they would have lived forever in their fallen state, and this would remain the human condition. There would be no possibility of redemption for us.
When God acts, even if it feels spoilsport-ish to our flesh, He's acting for our good. He really is.
Every motive, intention, and reason of God's is purely good at every level. Had physical death not been our penalty, we'd live forever as we are now--sinful world, crumbling bodies. Just like the seed in the ground, and just like Jesus our Prime Example, we have to die to live.
Notice there was no rule against eating from the tree of life before they sinned.
We may think the tree of life exits the narrative after Act 1, if you will. But no--not only does it reappear in Revelation (which many will realize), but it's a recurring motif all through Scripture. The earthly tree of life became lost to us. But the concept, and some of the comforts of it, have not.
Proverbs 3:18 says wisdom "is a tree of life to all who take hold of her."
Proverbs 11:30 says the fruit of the righteous is a tree of life, and he who is wise wins souls.
Proverbs 31:12 says, "Hope deferred makes the heart sick, but a longing fulfilled is a tree of life."
Proverbs 15:4 says, "A soothing tongue [speaking words that build up and encourage] is a tree of life."
How can we experience a tree of life now? By pursuing wisdom (which includes winning souls), by living according to the righteousness we have in Christ, by rejoicing in Godly longings fulfilled (answered prayer--and sometimes blessings we didn't ask for or know we needed!), and by speaking edification and encouragement to others.
So, we have the temporal, earthly tree of life and the metaphorical tree of life. But the real, lasting, heavenly one (the one on which the Edenic tree was modeled, I'm guessing) is in the Paradise of God, says Revelation 2:7. And if we overcome the world through believing that Jesus is the Son of God, we will get to eat from that tree.
If humans could play the long game even 1/1000 as well as God does, there's no telling how our lives would change.
But now, let's see the tree! It appears again in Revelation 22 (notice how the Bible pretty much opens and closes with the tree). The tree grows on either side of the river flowing from the throne, and it bears twelve kinds of fruit, yields its fruit every month, and the leaves of the tree are for the healing of the nations. Whoa. I remembered from several readings of Revelation that the tree bears fruit every month. What was harder to keep hold of, for me, was the fact that it produces twelve kinds of fruit. Twelve kinds of fruit and a fresh crop every month. I am a fruit lover. I cannot wait.
And the tree grows beside the river that flows from the throne of God and the Lamb (Revelation 22:1). Jesus is the source of all this.
Jesus is our Source. Our sin with a tree made Him die on a tree, and in the end, the tree of life springs forth from Who He is. If we wash our robes in His blood, believing and trusting in Him as the righteous who obey His commandments, we too will receive the right to eat from the tree of life (Revelation 22:14).