" It would not be possible to walk through a swamp like that. The branches grew so low.You could not make your way through the branches...Nick did not want to go in there now."
The swamp is a dark mysterious place in Nick's mind, an impenetrable force. This symbolizes the unknowns that Nick must overcome on his road to recovery.
"In the swamp the banks were bare, covered with cedar needles, the big cedars came together overhead, the sun did not come through except in patches...He did not want to go down the stream any further today."
Though Nick feels hopeless in many ways given his current circumstances, there is hope for a better future, hope to overcome his insecurities. He just needs to do it at his own pace.