Pray and thank God for what you see Him doing. Tell Him specifically what you are thankful for. John's comment - slam dunk. The church at Philippi was the only church that undertook Paul's support financially - a subject Paul addresses again in ch. By observing your lives I am convinced that God has started a good work in you and that He will keep on working until the day Christ returns.
Nothing brings more joy to the believer than changed lives.
When you get discouraged, remember that God never gives up on you. God finishes what He starts. When you're struggling in your life and you wonder how you can possibly face another day, just think that God is molding and shaping you into something beautiful for His kingdom. He who calls you is faithful, who also will do it . Paul's source of joy was to see God at work in the lives of his fellow-believers at Philippi.