Why did Harry utterly forget Sirius's mirror for months? [duplicate]

Question

In the fifth book of the saga, Sirius gives Harry a mirror so that he could communicate with him with ease without being intercepted. However Harry never thinks about it, even though the whole plot revolves around the need of communications between them. In fact if Harry remembered that mirror he almost certainly wouldn't have gone to the ministry and the whole ending of the book would be different, no Sirius death etc.

I know that when he received the mirror he didn't want to use it, fearing that Sirius would do something else. However:

1. Sirius explicitly told him that it was a mean of communication, and given that communication was so important in that book I don't buy that he absolutely never remembers that. They spent way too much time thinking about communication and Sirius to absolutely never think about that ever again.
2. The first time he goes to Umbridge's office he remembers of an other Sirius gift. Yet he isn't able to even think for a second about what he received the last time. This doesn't feel right. When I think about presents received from a person I usually remember more than exactly one. In fact given he takes weeks thinking about Sirius and taking to him, he must be really dumb to don't ever remember about the mirror.
3. When he finally remembers it's after Sirius death. And what he thinks is that he can communicate with the dead Sirius... When finding out that he is gone he never thinks holy duck! I just killed my godfather for nothing! It's all my fault! I should just jump of the castle and kill myself!. I myself would have been struck by the view of the mirror and start thinking how stupid I was etc, not simply let's try to communicate with him and when this fails nothing changed.

I find this is a huge plot hole. Even accepting that he forgot about it the whole time you cannot tell me that not having even the slightest feeling of guilt about this is normal. I mean everything that happened in the end, especially Sirius death, happened because he didn't remember about that. I'd be incredibly distressed by this.

Is Harry just really really dumb that he didn't even realize that it was all his fault? It's the only logical explanation I can find.

Ps: I know he feels some guilt about falling in Voldemort's trap, but he never seem to realize that if he thought about the mirror before nothing would have happened. in particular what troubles me is that the finding of the mirror after Sirius death doesn't change any of his feelings.

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Yet an other thing that doesn't seem to fit I'd that Sirius to seem to forget about that! I mean the first time Harry uses the Floo powder Sirius doesn't even take one second to think why Harry isn't communicating using the mirror. He never mentions it, yet he wastes time asking irrelevant things and telling Harry to study Occlumency. Even if he didn't have time then he ought to tell McGonagall or someone at Hogwarts to tell Harry to look at his gift. Surely the order had some mean of communication to provide a short generic message like that.

Moreover even if Harry hadn't realized what the mirror was, when one year later he finally understand he ought to have at least a thought about it... But nothing. This is to say, the answers to the related question aren't satisfying from this point of view.

Answer 1

To answer your points in order:

1. The text explicitly notes that Harry suspects that the form of communication is going to cause trouble, based on what he knows about Sirius, so he sets it aside for an emergency only. Given the other things Sirius has done in the past, the "form of communication" might have been putting a Dark Mark-type spell into the air that has a looping image of him saying, "Help me, Sirius. You're my only hope."
2. He's a teenager. Forgetting things happens. Plus, he might have still been thinking that the form of communication would cause more trouble than it would solve.
3. Harry's first reaction is hope (maybe I can still communicate with Sirius), then anger (Harry's prone to breaking things in a tantrum, not unusual for a developing boy struggling with testosterone), then disappointment (it "burned in his throat") followed by more hope (trying to get Nick to help him find Sirius's ghost). He then gets very depressed, "He walked slowly and miserably back up through the empty castle, wondering whether he would ever feel cheerful again", and is then distracted by helping Luna. He may have had many long and dark tea-times of the soul grappling with the choices he made, but he's a child soldier in a magical war, so he has to keep forging on.

In short, he's a kid who's going through a lot. Forgetting a present, and rapidly running through the emotional spectrum without learning beating himself to death over his choices is just normal.