"I have learned in whatever situation I am to be content" (Phil. 4:11).
Paul doesn't just say this, he says it from prison in Rome. Thus he proves that he has learned the lesson God has taught him: in whatever situation, to be content.
What does it mean to be content? In other words, the content of contentment. Using Philippians 4:4-13 as a guide, one definition could be this: a joy and peace not dependent on my situation, what I have or don't have, but on Christ who strengthens me.
If you have three minutes, watch this interview with a young man named Rob Jones and listen for him to say, in the Spirit of St. Paul, "I have an uncanny knack for being able to accept my situation."
And this: Are you asking God to change something in your life? Pray that prayer with faith, but a faith that gives God a choice: to change the thing, or to change the way you see the thing. Because it's possible that God wants to use the very thing you're asking Him to change "to advance the Gospel," the way He used Paul's imprisonment (Phil. 1:12).
Above all, ask God to teach you to be content. And then say, "I am learning in whatever situation I am to be content."