I honestly don’t know if I’ve ever actually written out my belief/practice on offering up one’s suffering in an official blog-post type way. I might have forgotten, but it bears repeating, so here goes.
The concept of offering one’s sufferings is one of the best things about Catholicism, in spite of those who, having been taught an abused notion of the concept, consider it “Catholic guilt,” or those who consider it as detracting from Christ’s sacrifice.
There are, as I have said before, two basic ways of looking at the Passion. Some Christians say, “Jesus suffered for me so I don’t have to.” Others of us say “Jesus suffered for me to give me an example.” I like to use the analogy of two heirs: One says, “Dad worked so hard all his life so I didn’t have to; I’d be dishonoring him if I slaved away needlessly.” The other says, “Dad worked so hard to provide me with security that I should work hard to provide my own children with security.”
It is a mistake to think that the Crucifixion is something “in the past.” It is the moment when God “died”. It is, with the Incarnation and Resurrection, one of the key moments when time and eternity intersected. God, who is Eternal, allowed Himself to die. God, who is eternal, and knows and remembers everything, allowed Himself to endure the worst tortures human beings can muster. Given His memory, He basically experiences that Moment always through Eternity.
(I proposed once that this is why God allows possession: so that, through the Rite of Exorcism, He can make the demons experience physical torments they would not otherwise endure).
Christ’s suffering is eternal and infinite, so that is why it can absorb the eternal suffering each of us deserves in Hell.
And so, when we are facing pain, we must remember what He did for us, and how we deserve every bit of pain we get for our most petty sins and our most egregious sins alike.
When I am about to be stuck by a medical needle, for example, I pray, “Dear Jesus, Your hands were pierced by nails, and You didn’t deserve it. I shrink because my hand will be pierced by a mere needle, and I deserve far more. I offer this pain in conjunction with your suffering on the Cross for _____[here I state my intention]”