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Your visa is expired and you should apply for a new one.
The enter before-date is what comes closest to the expire date, the meaning, quite obviously being that you must enter before the date given.
In a comment you mention the 10 year visa. While such a visa is available to US citizens, the respective agreement has only been put up in November 2014 so we can exclude that your visa issued in 2012 is a 10 year visa. You can recognize a Chinese 10 year visa from the issue-date to the enter before-date spanning 10 years.