Student or Learner
Is this sentence right?
I just realized I attached the 2015 December report in error in my earlier e-mail. Now I attach the January 2016 report that you requested.
I've just realized [that -- optional] I attached the December 2015 report to my earlier email. The January 2016 report you requested is attached. I apologize for the error.
Either 2015 December and 2016 January OR December 2015 and January 2016. Don't use one of each.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.