The Mirror Too Late

[D.G.]

No: HPFGUIDX 68746

At the very end of OoP, Harry discovers the mirror Sirius had given 
him, along with the instruction to use it whenever he wanted to 
talk to Sirius about something.  Am I missing something, or did Harry 
make a HUGE mistake in forgetting that he had this mirror in his 
possession?  He could have used it to keep in contact with Sirius 
during those weeks when Hogwarts was on security lock-down;  he 
could have used it to discover that Sirius was not at the Ministry 
being tortured by Voldemort's minions, and thus avoided the entire 
fatal battle at the end of OoP.  
  
Why doesn't Harry realize that he blew it?

D.G.