[The following information is from Wikpedia.]
This prayer is ascribed to either St. Bernard of Clairvaux, St. Gertrude or St. Mechtilde:
"O Loving Jesus, Meek Lamb of God, I, a miserable sinner, salute and worship the most Sacred Wound of Thy Shoulder on which Thou didst bear Thy heavy Cross, which so tore Thy Flesh and laid bare Thy Bones as to inflict on Thee an anguish greater than any other wound of Thy Most Blessed Body. I adore Thee, O Jesus most sorrowful; I praise and glorify Thee and give Thee thanks for this most sacred and painful Wound, beseeching Thee by that exceeding pain and by the crushing burden of Thy heavy Cross, to be merciful to me, a sinner, to forgive me all my mortal and venial sins and to lead me on towards Heaven along the Way of Thy Cross. Amen."
According to St. Bernard, he asked Jesus which was His greatest unrecorded suffering and the wound that inflicted the most pain on Him in Calvary and Jesus answered:
"I had on My Shoulder, while I bore My Cross on the Way of Sorrows, a grievous Wound which was more painful than the others and which is not recorded by men. Honor this Wound with thy devotion and I will grant thee whatsoever thou dost ask through its virtue and merit and in regard to all those who shall venerate this Wound, I will remit to them all their venial sins and will no longer remember their mortal sins."
According to Marie-Julie Jahenny (1850-1941), also known as "The Breton Stigmatist", Christ granted additional promises and graces to those who venerated this wound and spread the devotion.
A list of the promises in Christ's own words according to the visions of Marie-Julie:
1) “I will bless all the souls who propagate this devotion: I grant them abundant graces.” (March 29, 1878)
2) “O souls who love Me, who propagate this devotion, I take you under My protection, I keep you under the mantle of My affection.” (March 29, 1878)
3) “I will dispel the darkness that will come in their hearts.” (December 28, 1877)
4) “I will console them in their pains.” “I will come in the midst of their greatest afflictions, to enlighten, to comfort them.” (February 8, 1878) (December 28, 1877, February 8, and April 12, 1878)
5) “I will come to bless them in their undertakings.” (March 29, 1878)
6) “I will give them a tender love for the Cross. I will come to assist them at the time of death with this cross and I will introduce them into My Heavenly Kingdom.” (April 12, 1878)
7) “I will sweeten their agony.” (December 28, 1877) “I will come at the hour of death. I will console them in their passage.” (February 8, 1878) “Especially at the hour of death, I will come to give them a sweet moment of calm and tranquility. I will tell them: ‘O good holy soul, who has spread this devotion that I had so much at heart that it be made known, come to receive the reward of your labors, the fruit of blessing.” (March 29, 1878)
8) “I will shelter them, I will assist them, I will console all the souls that seek to propagate this Sacred Wound. At the time of death, I will console the souls that have compensated Me by their devotion and compassion to the Wound so deep and painful. I will come to strengthen them in their final fears. I will come and prepare their passage: Thank you, you who have compensated Me for My Pains.” (May 17, 1878)
9) “See,” Jesus said, pointing to His Wound with extreme tenderness, “all My children who have recognized this Wound, who have venerated It, have prayed to It, will have on the Last Day a great and generous reward. I do not simply show it, I pronounce it. My Word is Divine.” (May 1878)
(Note promise number 2, Our Lord said, “O souls who love Me,”. We must remember that those who love Him keep His commandments.)
Meditations we should reflect on while venerating the Holy Wound on His Shoulder:
1) The infinite generosity of Our Lord Jesus Christ for having willingly taken upon Himself our iniquities. (March 29, 1878) 2) His immense goodness in promising to carry our thankless sins on the road to Calvary. (December 28, 1878) He was overwhelmed, even to succumbing to the painful falls on the Way of the Cross. (February 8, 1878) 3) Marie-Julie: “Lord, it is our sins that You carry on this bloody and bruised Shoulder. It is our sins that make you suffer great incomprehensible pains as large as the sea. They are our sins that tore Your Sacred Flesh, which turned the Cross crimson.” 4) The infinite tenderness with which Our Lord Jesus Christ suffered this cruel Wound, which shows how heavy are our iniquities. Sympathize with this immense pain, so little known on earth. (December 28, 1877) 5) Our Lord invites us to contemplate the Wound so deep and painful, in which we have all participated. Ask for repentance and tears, gratitude and love. (February 8, 1878) It is there that our sins were drowned.
Marie-Julie: “When I see the blood that bathes the Cross, when I see it driven into this Wound, ah! Enormous weight of the Cross, how you make our Redeemer suffer! I can still see the Blood of my Divine king, marked on the paving stones of Jerusalem. Who would dare complain after such a love of Jesus crucified!” (May 17, 1878)