When the brothers return home with the bloodied coat of Yosef we read that,
לג וַיַּכִּירָהּ וַיֹּאמֶר כְּתֹנֶת בְּנִי, חַיָּה רָעָה אֲכָלָתְהוּ; טָרֹף טֹרַף, יוֹסֵף. לד וַיִּקְרַע יַעֲקֹב שִׂמְלֹתָיו, וַיָּשֶׂם שַׂק בְּמָתְנָיו; וַיִּתְאַבֵּל עַל-בְּנוֹ, יָמִים רַבִּים. לה וַיָּקֻמוּ כָל-בָּנָיו וְכָל-בְּנֹתָיו לְנַחֲמוֹ, וַיְמָאֵן לְהִתְנַחֵם, וַיֹּאמֶר, כִּי-אֵרֵד אֶל-בְּנִי אָבֵל שְׁאֹלָה; וַיֵּבְךְּ אֹתוֹ, אָבִיו.33 And he knew it, and said: 'It is my son's coat; an evil beast hath devoured him; Joseph is without doubt torn in pieces.' 34 And Jacob rent his garments, and put sackcloth upon his loins, and mourned for his son many days. 35 And all his sons and all his daughters rose up to comfort him; but he refused to be comforted; and he said: 'Nay, but I will go down to the grave to my son mourning.' And his father wept for him.
Rashi wonders why it is that Ya'akov refuses to accept the comfort and consolation of his remaining children? Why does Ya'akov never find consolation? Rashi explains
אין אדם יכול לקבל תנחומין על החי וסבור שמת, שעל המת נגזרה גזירה שישתכח מן הלב ולא על החי:A person cannot be consoled for someone who is living who he considers dead, for only on the dead was a decree issued that he should be forgotten from the heart, and not on the living.
The ability to forget is a great blessing indeed. Without it we would cease to be able to function in life, constantly consumed not only by grief for the inevitable losses each of us endure, but also by the shame of our sins, the guilt for our transgressions, and the constant pain of lost opportunities and missteps. For this reason, God blessed us (most of us at least) with the ability to forget; to move on, and somehow go on with life. So, if Ya'akov could not forget, even many years later, that must mean that somehow, something went wrong. For this reason, Rashi concludes that somehow Ya'akov knew that Yosef was still alive, and therefore he refused to accept the finality of Yosef's death.
Yet, Maharshal (quoted in Siftei Chachamim) adds another explanation, writing that, "When a person inflicts pain upon himself, he does not recognize that he is doing so excessively, for he considers this great amount of pain to be insignificant."
Put another way, Ya'akov didn't want to forget. He insisted on remembering Yosef, despite the pain that memory caused.
Truth be told, no parent can truly forget a child. The loss of a child remains with a parent in a way that no other loss can, leaving a deep scar, which might close over time, but never truly heals. Ya'akov's refusal to be consoled isn't out of the ordinary. Rather, he reacts as every parent does: life does go on, but the pain remains.