For this reason I ask you not to lose heart because of what I am suffering for you, which is your glory.
Paul said the suffering is not for himself, but rather for them. I believe he said that because he wants them to fix their gaze on the eternal, on what is permanent, things he had been writing about.
It is commonly noted that Paul wrote from prison here, a time of suffering. There could be an idea that if what he preached was so true about Jesus being the true ultimate king who is now alive, the God-man, why then did God allow Paul to be in prison, a man zealous for God, doing the right thing, and God all-powerful?
Paul was doing the right thing, but see what happened to him.
Why is Paul seemingly being punished for doing the right thing? Is his imprisonment not a judgment from God that Paul and what he proclaimed were wrong? So Paul took them through spiritual lessons to ground them in the spiritual understanding of things.
And had also focused them on the future:
He did this when he revealed to us the mystery of his will, according to his good pleasure that he set forth in Christ,toward the administration of the fullness of the times, to head up all things in Christ—the things in heaven and the things on earth.
And when you heard the word of truth (the gospel of your salvation)—when you believed in Christ—you were marked with the seal of the promised Holy Spirit, who is the down payment of our inheritance, until the redemption of God’s own possession, to the praise of his glory.
and he raised us up together with him and seated us together with him in the heavenly realms in Christ Jesus, to demonstrate in the coming ages the surpassing wealth of his grace in kindness toward us in Christ Jesus
The conclusion is that whatever Paul was going through does not contradict the glorious truth he had exposed, where Christ is presented as exalted.
Again, if He is so exalted, someone might ask, how and why is one of his foremost servants being crushed left, right, and center, and He did not do anything about it? Does that mean he does not care, or that he is not that powerful?
The answer, which we find in what Paul had been writing, is for them not to doubt the wisdom/power of God, which they have seen accomplish much already, foremostly through the death of Jesus, and for them to situate Paul’s suffering in that frame, in the context of the ongoing divine plan that nothing has stopped and nothing can stop.
Just as Christ’s suffering was for other people, Paul's was as well (2 Corinthians 12:15). The difference is that what Jesus did was in His suffering, whereas what Paul did led to his suffering, if we want to put it simply.
Paul does not want them to lose heart because of what he is going through; rather, he wants them to dig their root deeper into the soil of righteousness, the empowering soil of truth.
Even more than that, he wants them to see the value in his suffering for them. There is something about his suffering that benefits them. He did not say it is for their glory; it is their glory.
His suffering, Paul is saying, in my estimation, should be a source of strength for them, strengthening their faith as they are encouraged to stand firm and, in turn, become an encouragement to Paul in his suffering. In short, Paul’s suffering adds something to them. His suffering because they are one in Christ should reinforce their faith. That is the sense in which I am reading the idea that his suffering is their glory.
And he will immediately launch into a prayer for their strengthening (Ephesians 3:14-18).
James wrote that suffering serves a purpose:
My brothers and sisters, consider it nothing but joy when you fall into all sorts of trials, because you know that the testing of your faith produces endurance. And let endurance have its perfect effect, so that you will be perfect and complete, not deficient in anything.
Paul is constantly trying to re-frame his suffering. Suffering in the ancient world could be sold as a sign of defeat and ignominy. That is why people wonder: if it is difficult for the rich (which they might have thought was a sign of God’s favor) to enter into the kingdom of heaven, who can be saved? (Mark 10:26).
And it makes perfect sense to some that Jesus would not suffer death if God was truly with Him and could get Himself down from the cross (Matthew 27:40–43).
Paul is saying they should not feel defeated or retreat because of his suffering; don't let it overcome them emotionally, trample their courage, or move them away from their firm faith. It should actually accompany the opposite.
I want you to know, brothers and sisters, that my situation has actually turned out to advance the gospel: The whole imperial guard and everyone else knows that I am in prison for the sake of Christ, and most of the brothers and sisters, having confidence in the Lord because of my imprisonment, now more than ever dare to speak the word fearlessly.