and I trust in the Lord that shortly I myself will come also. (ESV)
I truly cannot imagine how it would feel to be in a prison cell. I can imagine that every moment of that time I would be thinking about being set free. The Apostle Paul was imprisoned in Rome. Truth be told, he had a much better attitude than I would have. In this verse he says that he "trusts in the Lord" that he will shortly come to see the church at Philippi.
I think the words "in the Lord" are key here. Paul does not know the future. But he is trusting in the Lord. In fact, the Apostle Paul is well aware that even if he dies in the prison cell, he will end up eternally free in Heaven. Paul chooses to experience the freedom that comes from knowing God, whether or not he is free in this life. Let's go back to some earlier verses in Philippians:
 I want you to know, brothers, that what has happened to me has really served to advance the gospel,  so that it has become known throughout the whole imperial guard and to all the rest that my imprisonment is for Christ. (ESV)
Paul knew that God was in control. He hoped in the Lord to be set free. Yet, he also knew that while he was imprisoned God would be glorified in his life. The imperial guard in Rome was hearing the Gospel because of Paul's imprisonment. Paul's will to be free was subject to God's will in all things. Paul had the right attitude.
How do you feel? Are you "imprisoned" in your circumstances? Ask the Lord to help you to trust in him. His will is best. Live the Gospel for those around you.