I hadn't noticed it mentioned in this thread, so I'll mention that the Psalms come to my mind in considering this question. The Christian Church made the Psalter its first prayer book and hymnal, and for many, it has always remained a primary source for Christian prayer and song.
A good number of psalms express frustration with God; charging God, so to speak, in regard to God's action (or non-action). These aren't the only word or the last word in their prayer vocabulary, but they're there, to be sure.
Jesus quoted one of them, of course: "My God, my God, why have you forsaken me?" (Psalm 22)
In our faith lives, we do not only feel joy, victory, and gratitude. It seems to me falsely empoverishing (and unbiblical) to not be allowed a way of prayer that expresses other darker, more difficult moods.
Whether we should feel certain moods is, in an a certain way, beside the point. In our living, real relationship, we are to be present with God (and realize that God is present with us) in all things.
In the pattern of Jesus, this includes our own very partial mirrorings of Gesthemane and the Cross.
Even unlike Jesus this includes, for us, our times of failure. This doesn't mean that sin isn't sin. It does mean that there's nothing "too far" for God. God really can "work all things together for good."
There is a spiritual danger, it seems to me, also, in only allowing the espression of the happy and positive. We can fall into the trap of trying to manipulate (if not bribe) God with what we deem to be the "appropriateness" or "faithfulness" of our responses.
So, it seems to be that Lewis, in A Grief Observed is offering something classic, appropriate. helpful, faithful--and godly.
Sorry to go on so long; I wanted to add this to the mix.
Regards.