What does Lewis mean by the things that stifle bitter resentment in elders in A Grief Observed In A Grief Observed on page 7 Lewis writes:


  Our elders submitted and said, 'Thy will be done.' How often had bitter resentment been stifled through sheer terror and an act of love — yes, in every sense, an act — put on to hide the operation?


What is Lewis getting at here? What does he mean by, "stifled through sheer terror," "an act of love," "hide the operation?" Any insights would be appreciated.
Section One deals with Lewis’s initial reaction to the death of his wife.  He asks this about God:


  Why is He so present a commander in our time of prosperity and so very absent a help in time of trouble?  I tried to put some of these thoughts to C. this afternoon.  He reminded me that the same thing seems to have happened to Christ: “Why hast thou forsaken me?”  I know. Does that make it easier to understand?”


Then comes the quote about “our elders” who submitted to God’s will and the question of bitter resentment being stifled, the whole thing “an act – put on to hide the operation” (page 8).

Further on in the book, in Section Two (page 29), Lewis likens grief to fear, suspense or waiting for something to happen.  It is in Section Three (page 45) where Lewis uses the metaphor of an amputation to describe how one never really recovers from the grief of the death of a loved one.


  To say the patient is getting over it after an operation for appendicitis is one thing; after he’s had his leg off it is quite another... Presently he’ll get back his strength and be able to stump about on his wooden leg.  He has ‘got over it’.  But he will probably have recurrent pains in the stump all his life... he will always be a one-legged man.


In Section Four Lewis realises that he is going through a process and likens it to a winding valley where any bend may reveal a totally new landscape.  He then introduces another metaphor to describe the process he is going through:


  Two widely different convictions press more and more on my mind.  One is that the Eternal Vet is even more inexorable and the possible operations even more painful than our severest imaginings can forbode. But the other, that ‘all shall be well, and all shall be well, and all manner of things shall be well’.


Lewis is moving through the process of grief, which is not unlike recovering from an operation.  He likens the death of a beloved to an amputation.

Perhaps now we can understand the reference to “the operation” given on page 8.  It is a metaphor that covers all sorts of suffering, and how Christians who came before, (“the elders”) the biblical saints, who submitted to the will of God and were persecuted and suffered for righteousness’ sake, struggled with internal doubts, resentment and fear.  “Why, God, are you allowing these terrible things to happen to me?”  The “operation” may be a reference to the price they pay for submitting to God’s will, just as Christ Jesus, who paid the ultimate price, suffered in order to do the will of his Father in heaven.

It is human nature to question and challenge what appears to be unfair, or unreasonable, or wrong.  This is where resentment is stifled and it may look to others that the believer who is suffering is simply putting on an act.  Just like grieving after the death of a loved one, all sorts of internal struggles are besetting other believers who did not fully understand why these dreadful things were happening to them (“the operation”) but who, nevertheless, endured.   You may find this article helpful:  https://en.wikipedia.org/wiki/A_Grief_Observed