Ephesians 4:31: "Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice."
Ah, bitterness. It's a poison, one we drink so often without realizing it. The world is full of well-wishers saying things like, "You deserve this!" or "No one should have to have this happen!" or "Look out for number one!" These and other like sayings are venom to our souls.
The Bible tells us "The wages of sin is death" (Romans 6:23). That is what we deserve. Jesus said, "In this world you will have trouble" (John 16:33). Why shouldn't we expect trouble to land at our door? Is Jesus a liar? Scripture says, "Do nothing out of selfish ambition or vain conceit. Rather, in humility value others above yourselves, not looking to your own interests but each of you to the interests of the others." In plain English: there's never a time we should be looking out only for ourselves.
Bitterness arises when we feel wronged. Sometimes we resent another person for the way they've treated us in the past. Paul goes on to say in Ephesians 4:32 to "Be kind and compassionate toward one another, forgiving each other, just as in Christ, God forgave you." The parable of the unmerciful servant reminds us of the great debt of our own that Christ's death canceled, and leads us to forgive one another as well.
To be bitter toward God means we don't believe He is loving, kind or just in His treatment of us. Perhaps we feel He's lost sight of us altogether.
The character and promises of God on the other hand, show us He keeps track of every hair on our head (Matthew 10:30), He's compassionate, as shown through Jesus, the manifestation of God the Father who was always compassionate, He's promised to take care of us (Matthew 6), to keep track of us (Isaiah 49:16), and to work all things for our good (Romans 8:28).
Bitterness is ugly for another reason, too. Hurt has us falling on our faces before God begging Him to pick us up, but bitterness has us shaking a fist at God demanding, "How dare you?" One is done in humility. The other is done in pride and scripture has nothing good to say about pride.
The Israelites often fell into bitterness, and that bitterness led to grumbling and complaining. Their memories were always short term, forgetting all the times God worked in their lives for their good.
When we don't understand why God isn't fulfilling our earthly wants, even godly wants like having a healthy family or finding a Christian spouse, or wanting to serve God in a certain capacity that God isn't allowing, we go back to God's character and promises. We lean into those instead of our own sinful and flawed thoughts and emotions, and trust God sees us, knows us, and is working for our good. Then we can "get rid of all bitterness" and we remember all the times in the past He bailed us out, took care of us, maneuvered our lives for our benefit.