I don't think it was a case of him "forgetting" about it. but JKR's attempt at suspense. Giving Harry almost no way of confirming the memories. I'm sure it was just one of the many things that are "forgotten" over the course of the books. We all wanted to see more interaction between Harry and Sirius, it was really a short lived realtionship.
When Sirius first gave Harry the mirror, Harry vowed never to use it, because 'it would not be he that lured Sirius from his place of safety.' .. Consequently, he never even unwrapped it. He didn't even know what it was, and simply buried it in his trunk, forcing from his mind. After Sirius' death, he unwrapped the package finally. Upon figuring out that, using the mirror, he COULD have contacted Sirius, and seen through the trap, he was furious, and shattered the mirror in spite and rage.
Well said. But I don't think it was spite, more rage and grief. He tried, momentarily, to call Sirius first.