The greatest and most protracted spiritual suffering arrives with the withdrawal of grace,
as part of the very positive "discipline" of those God is training as his own sons and daughters. Death is God''s justice and is meant to bring us to our senses and repentance. The withdrawal of grace is a death meant to chastise and correct - and is God version of tough love. Archimandrite Zacharius, in his "Christ Our Way And Our Life" lays it out beautifully. Grace makes our prayer and spiritual inner life wonderful, but when it is withdrawn, it all becomes empty and a labor and not satisfying. God can even seem to despise us at times, yet rebellion we know must not happen. Trapped again with no heart to keep trying, the soul must act on the little grace it is experiencing, and repent even more deeply until grace restores the souls right order. The Fathers say that when grace is present and the soul feels rightly ordered, realize it won't last, and during the suffering of His withdrawal, realize it will also end and grace will be restored. Zacharias says grace withdraws because of our sins of pride and presumption, but can also be for teaching an important truth, or for testing us, as gold is tested by the refiner, etc. The soul must always try to accept this divine way of leading us as beneficial and rooted deeply in love - without any rebellion - and with an even deeper admission of our weakness, which is right up there on the big screen when grace "leaves."
Anyone who has not experienced this will have difficulty understanding it. If they move their attention to the heart, they will, in time enter into it. This suffering is increased because love is increasing, and with it our longing for his intimate embrace which seems so distant when grace withdraws. I am happy to share about suffering, but since others have written widely on this subject I will stop here unless you have questions☺