Yet Paul says that by his sufferings he is filling up a gap, something that is missing, and he is doing so on behalf of the Church.
That missing thing is the distribution of God's grace and of the salvation of Christ to all people. Many people have never heard the Gospel. Others have received it only partially and don't fully understand it. Others have been misinformed by poor teaching. Still others have rejected the teachings of Christ and risk eternal damnation. Even committed, devout Christians sometimes lose their intimacy with God through sin and negligence and need a fresh outpouring of grace. For all these people, Paul offers his own suffering that God's Word might be complete in them, that it might reach its fullness in their lives and lift them up to a close relationship with God and eventually to Heaven.
This whole process is called subjective redemption. Let's define the term by contrast first. Objective redemption refers to the redemption won for us by Christ. This is a grace, a gift from God. Jesus died on the cross to obtain this redemption. We do nothing to earn it; our job is to accept it through a living faith. Subjective redemption, on the other hand, refers to the distribution of the saving grace won for us by Christ. Here's where we come in. Through our prayers and the sufferings we patiently offer to God, we can help distribute divine grace to those around us or even to people we don't know. God lets us help. He wants us to ask Him to shower His grace on others. He wants us to offer up our trials and struggles that other people might benefit from them. He doesn't need our prayers or sufferings, of course, but He wants us to participate and take an active role in spreading redemption throughout the world.
Therefore, like Paul, we fill up in our own flesh, in our suffering and prayers, what is lacking in the afflictions of Christ, that missing link of distributing God's saving grace through the entire world, to every person. We know the "mystery hidden from the ages and from generations," for it has been made manifest to us through the Gospel. God has made known to us the "the riches of the glory" of Christ. Our job is to proclaim those riches to others and to allow God to use us to help bring His children to the perfection of Heaven.