Does it sound like there is a lack of peace on David's part here? All the fighting men of Israel were panicked with fear because they only saw the giant while David saw all God had already done for him:
But David said to Saul, "Your servant has been keeping his father's sheep. When a lion or a bear came and carried off a sheep from the flock, I went after it, struck it and rescued the sheep from its mouth. When it turned on me, I seized it by its hair, struck it and killed it. Your servant has killed both the lion and the bear; this uncircumcised Philistine will be like one of them, because he has defied the armies of the living God. The Lord who rescued me from the paw of the lion and the paw of the bear will rescue me from the hand of this Philistine." - 1Samuel 17: 34-37 (NIV)
Beloved, the Lord who rescued you from one situation is the same one that can deliver you from whatever you face. If you are thinking He has never rescued you from the bear or lion then you are forgetting your salvation! Once we were headed for eternal destruction but now we enjoy eternal life! Once we were walking in utter darkness but now we walk in the light of Christ! There is not giant greater that we will face and God already has proven that He is God! Thanksgiving in prayer reminds us exactly who we serve. Secondly however, thanksgiving acknowledges that what we pray is already accomplished if we are in the will of God. It serves to strengthen our faith.
"Have faith in God," Jesus answered. "Truly I tell you, if anyone says to this mountain, "Go, throw yourself into the sea,' and does not doubt in their heart but believes that what they say will happen, it will be done for them. Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours. - Mark 11: 22-24 (NIV)
When we boil down a lot of our situations that lack peace we will find that we are actually lacking faith in who God is in our lives. When we have no peace about our job situation we might be lacking faith in God to provide. When we have no peace about our medical report we might be lacking faith in God as our healer. The Bible is filled with the promises of God and He can never deny Himself. The enemy whispers lies into our lives at our most desperate situations. Those lies are always in contradiction to the promises of God. A lack of peace indicates that we might be believing the lies over the promises. Thanksgiving in prayer acknowledges that God will perform that which He has promised.
God's peace plan for your life starts with joy. It starts with rejoicing in the Lord always. This creates harmony in our lives with the God we serve that is difficult for the world to infect. It continues with letting the fruit of gentleness be evident in everything we do. This limits the amount of drama we invite into our lives. We must always remember that the Lord is near. He is not some distant being but a friend that sticks closer than a brother, always at our right hand if we would gain an eternal perspective in our lives. The peace plan is centered on our prayer life with God. It must be a prayer life that encompasses all of our life, not just the parts we want to bring to God. Whatever we hold back from God will be where we find ourselves lacking peace. It must be a thankful prayer life that remembers all God has already done and is confident that He who started a good work in us will complete it until the day of Christ Jesus. Never forget what is at stake in this plan. Not just any old peace. Not the fleeting peace the world offers. No. The peace of God is at stake.
Peace I leave with you; my peace I give you. I do not give to you as the world gives. Do not let your hearts be troubled and do not be afraid. - John 14: 27 (NIV)
It is a peace that literally transcends anything any human being can even begin to understand! Think about that for a moment. Look, no one can possibly walk in that peace 24-7. Our flesh and the world are constantly fighting against us and the enemy is always on the prowl. That does not change the fact that the peace of God is available if we would make ourselves available to God's plan. It will guard our hearts and minds in Christ Jesus. Just stick to the plan beloved.