going back and forth：为一固定用法,意思是“来来回回”；加上定冠词 就是一个“名词短语”,要作为一个独立的名词看待.即：the going-back-and-forth .
We knew（that） the going-back-and-forth wasn't going to cut it any more.
中间省略了that ,后面是宾语从句,the going-back-and-forth为宾语从句的主语.
cut：在这里为：“与…相交；与…相切；穿梭”的意思.
We knew the going back and forth wasn't going to cut it any more.
“我们知道：不能再这样来来回回继续穿梭下去了.”
直译就是：
我们知道：这种“来来回回”再不要继续穿过来穿过去了.
it =the going-back-and-forth
明白了吗?