At first glance, the beginning of verse 9 reads like a contradiction; how can the psalmist call God his rock, then immediately express he feels forgotten? Calling God his rock would suggest comfort, security, or a safe place, yet he feels alone. Digging deeper, it makes sense. It is because God is his rock that he can go to God with his raw thoughts and feelings.
There is nothing we feel, say, or do that separates us from God (Romans 8:39). He welcomes our honesty because that is where true intimacy and relationship with Him is. Can you imagine going through hardship and not being able to vocalize that struggle with God? There is even a comfort just in knowing that God welcomes our vulnerability and emotions, no matter how good or bad they may be.
Psalm 42 closes with an encouragement and challenge: even when we feel downcast and in despair, we can and should put our hope in God, the only one that can truly satisfy.