"Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ's afflictions for the sake of his body, that is, the church, of which I became a minister according to the stewardship from God that was given to me for you, to make the word of God fully known..." - Colossians 1:24-25 ESV
This is a very intense passage.
The Greek word for 'sufferings' is pathēma (Strong's G3804) which is derived from pathos. To understand this term we must look briefly at Aristotle.
Aristotle said that there were 'three ingredients for persuasion' or 'appeals'. Ethos, pathos and logos were identified as those three. They are also referred to as the 'three artistic proofs'. Ethos stands for authority. It is when a speaker gives credentials so that the hearer will believe and the speaker is qualified to say something authoritative on a subject. Pathos is an emotional appeal and logos is a logical appeal. Paul was well versed in Greek thought but the scriptures were not wriiten by Paul. God used Paul to pen the scriptures.
We see here and intertwining of the Holy Spirit and Paul's living human spirit. Paul agonizes in his body. He is in prison. This is one of the four books he writes from prison. His body aches. He experiences loneliness. He has longings for light and freedom and comfort. During this time the Spirit is convincing Paul of Christ's sufferings on the cross. The phrase 'filling up what is lacking' is antanaplēroō (G466) is very close to the idea of persuasion. When you convince someone of something you are 'filling up what is lacking' in their understanding.
Paul is saying, 'I am glad I understand what Christ did on the cross. He is persuading me and so I in turn, want to persuade you.'
I don't think we appreciate the value of agony. Perhaps because we don't want to appreciate it. Our minds shy away and want to hide from pain.
I don't believe Paul was claiming to be adding anything to Christ's suffering. I think Paul was identifying with it and there was a deep persuasion happening in his heart that he wanted to pass on to the Colossians.
"...the mystery hidden for ages and generations but now revealed to his saints. To them God chose to make known how great among the Gentiles are the riches of the glory of this mystery, which is Christ in you, the hope of glory. Him we proclaim, warning everyone and teaching everyone with all wisdom, that we may present everyone mature in Christ. For this I toil, struggling with all his energy that he powerfully works within me." - Colossians 1:26-29 ESV
The cross was planned out from before time but now it is revealed and Paul's sees his time as a time of opportunity. Even though he is in prison he has been persuaded and the Spirit of God communicates through Paul the 'rich glory' of the mystery which is now being revealed.
Christ in us is the mystery. The invisible God being made visible through sometimes suffering humanity. God wants to woo the world and He has chosen to use us to do that.
You might feel trapped. You may have been deeply hurt. Jesus was all of those things and more. There is an energy working in the one who suffers and still believes. God is going to persuade people with the message when it comes from that kind of messenger.