So we beat on boats against the current borne back ceaselessly into the past. meaning

The sentence explains a time of struggling when we get nothing at the end and are brought back to our past or near past. Like a boat which is continuously rowed but is found at the same place at the end of the day.

Explanation:

This refers to the dualities of Gatsby and America. Nick is saying he will get into Gatsby's boat, the one that only sailed backwards on the sea of history--into the past to recapture his childhood dreams.

This book was written in 1922 and accordingly it was a time of struggling. In the boat rowing a continuous struggle is required to move it forward, likewise a continuous struggle was required in 1922 to improve the country's situation. But everything seemed useless as the struggle took them to their past to a place from where they had begun.

Its the first one

No matter how hard we try, we can never escape our pasts.