I think this reflection on the fourth Sorrowful Mystery of the rosary explains well how the sufferings of the Mystical Body of Christ (those of us who belong to the Catholic Church) can be united with those of Christ Himself. As His own Body, we bear His own cross, when we willingly allow ourselves to suffer as God asks of us in order to accomplish God’s will and man’s salvation:
“4. Jesus Carries the Cross
“Let us meditate upon Jesus Christ on the way to Calvary laden with His cross. He falls under the weight of this burden. To expiate sin, He wills to experience in His own flesh the oppression of sin. Fearing that Jesus will not reach the place of crucifixion alive, the Jews force Simon of Cyrene to help Christ to carry His cross, and Jesus accepts this assistance.
“In this Simon represents all of us. As members of the Mystical Body of Christ, we should all help Jesus to carry His Cross. This is the one sure sign that we belong to Christ[ ]if we carry our cross with Him.
“But while Jesus carried His cross, He merited for us the strength to bear our trials with generosity. He has placed in His cross a sweetness which makes ours bearable, for when we carry our cross it is really His that we receive. For Christ unites with His own the sufferings, sorrows, pains and burdens which we accept with love from His hand, and by this union He gives them an inestimable value, and they become a source of great merit for us.
“It is above all His love for His Father which impels Christ to accept the sufferings of His Passion, but it is also the love which He bears us.”
(q. from Dom Columba Marmion, O.S.B., Abbott of Maredsous, “The Mysteries of the Rosary,” at http://www.ewtn.com/library/Prayer/MYSTROSA.HTM)