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:
calledtocommunion.com/2009/08/a-catholic-reflection-on-the-meaning-of-suffering/
This article quotes from Pope Pius XII, who wrote:
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.
Mary