Note: As a personal, spiritual discipline, I write a prayer for each of the forty days of Lent; each petition focusing on a theme, truly, relating to a care or concern weighing on my mind and heart, at times, vexing my soul and spirit…
On what is lacking in Christ’s afflictions: O Jesus, oft I have pondered Your Apostle’s word, wondering what would…what could be lacking in Your suffering for my sake; and, in my wondering, worrying that were there to be something lacking, making Your sacrifice somewhat, somehow less efficacious, then where would I be, who would I be without the grace of Your salvation?
Then I recalled what Your Apostle hath written in another place about my being a child of Abba, Your Father, my Father, and thus an heir with You in salvation providing that I, too, suffer with You for the sake and in the service of others.
Ah, O Jesus, is this, at least, in part, Your meaning of taking up my cross and following You?
I pray You, O Jesus, speak to me through Your Spirit that I may know the truth of it. Amen.
 See Colossians 1.21-24 (my emphasis): And you who were once estranged and hostile in mind, doing evil deeds, he has now reconciled in his fleshly body through death, so as to present you holy and blameless and irreproachable before him; provided that you continue securely established and steadfast in the faith, without shifting from the hope promised by the gospel that you heard, which has been proclaimed to every creature under heaven. I, Paul, became a servant of this gospel. I am now rejoicing in my sufferings for your sake, and in my flesh I am completing what is lacking in Christ’s afflictions for the sake of his body, that is, the church.
See Romans 8.15b-17 (my emphasis): When we cry, “Abba! Father!” it is that very Spirit bearing witness with our spirit that we are children of God, and if children, then heirs, heirs of God and joint heirs with Christ; if, in fact, we suffer with him so that we may also be glorified with him.
 See Matthew 16.24, Mark 8.34, and Luke 9.23. See also a-Lenten-prayer-a-day, day 27, On following Jesus and bearing my cross