Colossians 1 24 Now I rejoice in what was suffered for you, and I fill up in my flesh what is still lacking in regard to Christ’s afflictions, for the sake of his body, which is the church. 25 I have become its servant by the commission God gave me to present to you the word of God in its fullness— 26 the mystery that has been kept hidden for ages and generations, but is now disclosed to the saints. (NIV)

“What was suffered for you” = “for your benefit”, NOT “in your place”
Fill up = complete / participate in
Christ’s afflictions = a word for trials / sufferings which is not used anywhere to refer to the cross
For the sake of his body, relates to Paul’s suffering for the church, NOT to Christ’s suffering.
24 I am happy in my sufferings for you. There are things that Christ must still suffer through his body, the church. I am accepting, in my body, my part of these things that must be suffered. (New Century Bible)
24 I am glad when I suffer for you in my body, for I am participating in the sufferings of Christ that continue for his body, the church. (New Living Translation)

Christ’s death is central to our salvation
God’s holiness and justice demand punishment for human sin. Christ died as a sacrifice to pay that penalty for sin which we could never pay. The whole of the Old Testament system of sacrifices foreshadowed Christ’s death. Christ died in our place – penal substitution.
Christ’s death is vicarious – for our benefit Mark 10:45
Christ’s death is representative – He repents and obeys on our behalf 2 Corinthians 5:19-21
Christ’s death is sacrifical – voluntarily offering Himself for our benefit Isaiah 53:10-12
Christ’s death is atonement – the sacrifice offered to take sin away 1 Peter 2:21-25
1 Peter 1:18-25; Romans 3:22-25
Christ’s death is “expiation” – dealing with the consequences of sin in our lives Hebrews 2:14-17
Christ’s death is “propitiation” – dealing with God’s wrath against sinners 1 John 2:1-2
Christ’s death is substitutionary – in MY place Isaiah 53:6-7 1 Peter 3:18
It is a Roman Catholic error to suggest that Christ’s sacrifice needs to be completed by the suffering of the saints. Christ’s sacrifice is entirely sufficient ! 1 Corinthians 1:13; Colossians 2:11-15

In comparison to all these aspects of Christ’s suffering, Paul’s suffering can only be vicarious.
As Christians we are united with Christ. Christ’s sufferings with His people continue. Acts 9:4-5
So how can our sufferings bring blessing to others?
Ephesians 3:13 2 Corinthians 1:3-7 2 Corinthians 4:7-12