Looking back on King David's life we know that God had delivered him from much anxiety, sorrow, pain, anguish, suffering, agony, torment, heartache and heartbreak. These are many of the same emotions we all experience in life at one time or another. We can trust God as David did with all of our emotional stresses. If anyone had a right to nurse a grudge it was David towards Saul who attempted to kill him. He trusted God with his life and knew that it would only do harm to himself and God's purpose in his life.
The root of anxiety and stress in my own life is always my own lack of faith. I am constantly repenting to God for my unbelief. Sure I have blamed a million and one other things or other people but when I finally am honest with myself it is always by my own doing, my own sin, my unbelief that God has taken care of everything for me.