This is where the Communion of the saints come in. We can bear sufferings for each other and by doing so, we participate in the redemption of Christ.
Here is a very good article about suffering:
This article quotes from Pope Pius XII, who wrote:
Moreover as our Savior does not rule the Church directly in a visible manner, He wills to be helped by the members of His Body in carrying out the work of redemption. That is not because He is indigent and weak, but rather because He has so willed it for the greater glory of His spotless Spouse. Dying on the Cross He left to His Church the immense treasury of the Redemption, towards which she contributed nothing. But when those graces come to be distributed, not only does He share this work of sanctification with His Church, but He wills that in some way it be due to her action. This is a deep mystery, and an inexhaustible subject of meditation, that the salvation of many depends on the prayers and voluntary penances which the members of the Mystical Body of Jesus Christ offer for this intention and on the cooperation of pastors of souls and of the faithful, especially of fathers and mothers of families, a cooperation which they must offer to our Divine Savior as though they were His associates.
The Catholic Church is the only church which truly understands the redemptive nature of suffering. We are the only Church who look at the Cross and don’t see just Jesus’ suffering, but we see our Lord redeeming the world. And through his love for us, he has made it possible for us to be a part of the redemption of the world.
That, I believe, is what Our Lord was telling Sister Faustina. We should never “waste” suffering, but always offer it for redemption of others through the sacrifice of Jesus Christ.