According to Freud, the uncanny is the part of ourselves that we repress. If we don't deal with it, it will keep popping up unexpectedly to confront us. Because we have repressed a part of ourselves, its appearance can seem weird or eery, and, if we are too repressed, this can interfere with our normal functioning.
One could argue that the part of himself that carelessly killed the albatross is the part of himself the Mariner represses: the very carelessness of this aggressive, angry acting out shows it is something the Mariner wants to block out of his conscious thoughts, almost as if he hadn't real done it. The Mariner's self concept doesn't want to acknowledge his aggressive, even animalistic, and amoral murderous drives. In his perfectly arranged world of repression, he could "forget" that his killing of the albatross ever happened. He could happily go on with this life, never having to face his demons.
But it could be argued that his repression has become too great. The symbol of the "return of the repressed," the albatross, won't go away. Killing the albatross troubles his life and makes it uncanny. For example, try as he might, he can no longer find nurturance in the ordinary staples of life, such as water, finding that although water is everywhere, he cannot find a drop to drink:
Water, water, everywhere,
Nor any drop to drink
Everything in his everyday life is poisoned. He is "stuck," unable to grow psychologically because of the parts of himself he refuses to deal with. In fact, the image of a "painted" ship and a "painted" ocean show how uncanny or weird his normal world has become:
Day after day, day after day,
We stuck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.
The therapeutic moment occurs when he accept the repressed parts of himself and makes the decision to take responsibility for his actions and initiate change. As the Mariner embraces loving "all things both great and small," even including his repressed self, he comes to healing.