The following is an excerpt from The MacArthur New Testament Commentary on Philippians 4.

I can do all things through Him who strengthens me. (4:13)No matter how difficult his struggles may have been, Paul had a spiritual undergirding, an invisible means of support. His adequacy and sufficiency came from his union with the adequate and sufficient Christ: “I have been crucified with Christ; and it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me and gave Himself up for me” (Gal. 2:20).

When Paul wrote I can do all things he had in mind physical, not spiritual things. Ischuo (I can do) means “to be strong,” “to have power,” or “to have resources.” It is variously translated “overpowered” (Acts 19:16), “prevailing” (Acts 19:20), and “effective” (James 5:16). The Greek text emphasizes the word translated all things (a reference to physical needs; cf. vv. 11–12) by placing it first in the sentence. Paul was strong enough to endure anything through Him who strengthen[ed] him (cf. 1 Tim. 1:12; 2 Tim. 4:17). The apostle does not, of course, mean that he could physically survive indefinitely without food, water, sleep, or shelter. What he is saying is that when he reached the limit of his resources and strength, even to the point of death, he was infused with the strength of Christ. He could overcome the most dire physical difficulties because of the inner, spiritual strength God had given him. In the words of Isaiah,  He gives strength to the weary, and to him who lacks might He increases power. Though youths grow weary and tired, and vigorous young men stumble badly, yet those who wait for the Lord will gain new strength; they will mount up with wings like eagles, they will run and not get tired, they will walk and not become weary. (Isa. 40:29–31)Perhaps the clearest illustration of this truth in Paul’s life comes from 2 Corinthians 12:7–10:  Because of the surpassing greatness of the revelations, for this reason, to keep me from exalting myself, there was given me a thorn in the flesh, a messenger of Satan to torment me—to keep me from exalting myself! Concerning this I implored the Lord three times that it might leave me. And He has said to me, “My grace is sufficient for you, for power is perfected in weakness.” Most gladly, therefore, I will rather boast about my weaknesses, so that the power of Christ may dwell in me. Therefore I am well content with weaknesses, with insults, with distresses, with persecutions, with difficulties, for Christ’s sake; for when I am weak, then I am strong.Paul was tormented by a “thorn in the flesh,” most likely a demon who was behind the false teachers tearing up his beloved church in Corinth. This was the worst of all trials for him, because of his “concern for all the churches” (2 Cor. 11:28). He repeatedly begged the Lord to deliver him from the torment of that demonic attack on the church. But instead of delivering him, the Lord pointed Paul to the sufficiency of His grace. Contentment comes to believers who rely on the sustaining grace of Christ infused into believers when they have no strength of their own. In that sense, contentment is a by-product of distress.