8 Indeed, I count everything as loss because of the surpassing worth of knowing Christ Jesus my Lord. For his sake I have suffered the loss of all things and count them as rubbish, in order that I may gain Christ 9 and be found in him, not having a righteousness of my own that comes from the law, but that which comes through faith in Christ, the righteousness from God that depends on faith— 10 that I may know him and the power of his resurrection, and may share his sufferings, becoming like him in his death,
10 I rejoiced in the Lord greatly that now at length you have revived your concern for me. You were indeed concerned for me, but you had no opportunity. 11 Not that I am speaking of being in need, for I have learned in whatever situation I am to be content. 12 I know how to be brought low, and I know how to abound. In any and every circumstance, I have learned the secret of facing plenty and hunger, abundance and need. 13 I can do all things through him who strengthens me.
Oftentimes Philippians 4:13 is used in a way whereby a person strives to convey that they can do anything with their sports career, goals in life, marriage, etc. through God who gives them strength. While some may be well intentioned in honoring the Lord Jesus in those things, Philippians 4:13 is not about that. Rather it is Paul conveying how he can face bleak circumstances and advantageous circumstances. How Paul can in all of life's trials and tribulations be content, which in this text means satisfied in the Triune God. Contentment for Paul was being happy in the Lord Jesus Christ. We see the context being contentment in verse 11 when Paul states that he has "learned" in all situations "to be content". Paul then summarizes all the circumstances and conditions he had found himself in up to this point in verse 12.
The secret to Paul's contentment in Christ was God's power at work in Paul. Paul in Philippians 1:21 expounded to the Philippians that for him "to live was Christ" and in Philippians 3:8,10 that his aim in life was to know Christ. Paul wanted to know Christ and find his satisfaction in Christ. In Philippians 4:12 we see that contentment (satisfaction in Christ) was something Paul grew in and learned as a believer. Paul knew that God's grace was sufficient. That Christ was more than enough in all circumstances.
Paul states this same theme in 2 Corinthians 12:7-10 when he discusses his thorn that he asked God to remove. God rejected all three requests telling Paul that His (God's) strength was sufficient for Paul in the affliction. Paul's states his outlook after the interaction with God in prayer in 2 Corinthians 7:10, "10 For the sake of Christ, then, I am content with weaknesses, insults, hardships, persecutions, and calamities. For when I am weak, then I am strong."
Paul found his happiness and satisfaction, by the power of God, in Christ alone. Only in Christ alone can we find lasting happiness and true contentment. Where do you derive your contentment, your happiness? Circumstances or Christ? Only one satisfies.