After the children of Israel passed through the Red Sea they became thirsty in the wilderness. They found water at Marah but it was too bitter to drink. The people complained to Moses who prayed to God. The Lord showed Moses a tree and when Moses threw it into the water the water became sweet (Exodus 15:22-25). The tree is a type of the cross. God here is foreshadowing the power of the cross to remove bitterness.
It is certainly true that the cross removes bitterness from eternity. Jesus described the rich man in Luke 16 tormented in the flame longing for just a drop of water from the tip of Lazarus's finger. What a dramatic contrast to those who through the power of the cross enter the New Jerusalem where the water of life in a pure river, clear as crystal, proceeds from the throne of God and of the Lamb (Rev. 22:1).
Not only does the cross remove bitterness from eternity, the cross also removes bitterness from this life. And since this event is after the Red Sea experince, a type of salvation, perhaps this is the best way to understand this passage. Life can get polluted with opinions, convictions, expectations, traditions, likes and dislikes. These impurities can rob us of joy when things don't go the way we want. The unpleasant result is bitterness. The solution is the cross where we die to our values, persuasions, comfort, and sense of right and wrong.
Are you cross-eyed?
Place the cross between you and every frustration. Don't focus on the problem, focus on your cross. As you become "cross-eyed" life's bitterness passes and life becomes sweet again.
The cross of Jesus Christ is two-sided. One side is for Jesus and the other is for His followers. Jesus died to take away our bitterness and give us a sweet eternity. We also must die on the cross in order to overcome life's bitterness and enjoy a sweet life now.