So we beat on....

In my opinion, Nick is touching on the famous saying of in which Gatsby says you can repeat the past. At first Nick doesn't agree with Gatsby that the past can be repeated, by it seems he has come to new conclusions by the end of the book. In my own summary form I would say that the quote means that people just  keep going on, pushing against the many trials of life, continuously turned back into their own past. It's like a person is walking in a rut, and you can't change the rut of the path, only keep going forward or falling backwards to walk the path again.