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I can’t give a full answer to this, because it is relevant to books six and seven. However, the short answer is that Harry was determined never to use the mirror, as is clearly stated in chapter 24: ‘he knew he would never use whatever it was’. For once in Harry’s life, he does not succumb to curiosity, he hides the mirror and the temptation away from himself, and then, when it might have been useful, he has forgotten it.
The mirror might not have helped as much as you think, but on the other hand, will help more than you think. You’ll have to read the final books to understand that!