big on looking back的用法和样例：
- Every day is more difficult to endure than the last, but, on looking back, one rues the shortness of the time.
- On looking back it seems that the path to commercial success with fluidized processes is unusually painful and complex.
- Life is like walking across a suspended tightrope. If one does not concentrate on looking ahead, but keeps looking back instead, one will surely fall.
- Life is like walking across a suspended tightropoe. If one does not concentrate on looking ahead, but keeps looking back insdead, one will surely fall.
- I still shudder when I look back on the past.