A/N This will be AU from the end of fourth year (the earlier years are just like the books)
"Ah, Severus, how goes it with Mister Potter? Progress I hope." Albus Dumbledore asked as his potions professor stormed into his office.
Snape smiled almost cruelly, "Oh, the boy will never be able to learn to be an Occlumens, Albus. And this time you can't claim it is merely my hatred for him that is holding him back." He seated himself before Albus in one of the office chairs without being invited. "He has a block somewhere in his subconscious that is actively preventing him from learning. Until it is removed he won't even be able to learn to even clear his mind."
Dumbledore's eyes widened slightly, "A Block? And it is actively preventing him, not just passively?"
Snape nodded the regal look both augmented and spoiled by his sneer. "Do you doubt my abilities to detect a block?"
"No, no. Not at all, Severus, I suppose I was merely hoping that it could be something else." He paused, pondering what it might mean for young Harry to have a block in his mind and memory. "Could you determine anything from the block? How old or how extensive it is perhaps?"
Snape's smile finally gave away to a fierce scowl. "No, I was unable determine anything about it, other than it is extremely strong, that is one of the reasons I have come here, I shall need your assistance if I am to break it."
"Of course, Severus, if you and Mr. Potter are not averse, I think it shall be best if we try to remove it immediately." Dumbledore said as he stood to move around his desk.
Severus nodded curtly, "I assumed as much, Headmaster. I have called Poppy to come and have left the boy in the hospital wing." Snape stood as well following Dumbledore as he weaved his way through the cluttered office, down the spiral staircase, and passed the gargoyle at the base.
They made their way towards the hospital wing tensely, only the shuffle of robes and tapping of their shoes disturbing the silence. Finally Dumbledore stopped before the hospital doors and turned to face Severus, "Have you explained to Harry what is going on, yet?"
Snape sneered, "The boy will trust your judgment. He need know no more." With that he slid around Dumbledore and pushed wide the doors making his way into the hospital wing towards the anxiously waiting Poppy Pomphrey and Harry Potter. Dumbledore followed a step behind him with a sigh.
"Dumbledore," Poppy exclaimed, "what is it now that I must be called from St. Mungos during the break?" Her gaze inspected both Dumbledore and Harry, almost as if expecting one of them to sprout a grievous wound any moment.
"If we might all take seats I shall explain to both you and young Harry here what has brought us together today." As he spoke he drew his wand from his robe and flourished it lightly and four of the chairs around the room instantly flew towards them, settling a few feet away in a comfy circle. Even as they all moved to take their seats Dumbledore twitched his wand again and his chair grew large bright purple cushions with small yellow ducks swimming around it. The slightly twitch of amusement his moth made at the others' looks of amusement and exasperation was hidden by his beard. "Now, Severus has told me that while he was trying to teach you Occlumency he discovered a block in your subconscious that is preventing you from learning how to clear your mind and eventually become an Occlumens. He brought you here so that he and I might remove this block from your mind." In typical Dumbledore fashion he said all this as if commenting on the strange cloud pattern that he'd seen a few days ago.
"Um, Professor, if I've got a block maybe we shouldn't get rid of it, I mean what if my mind blocked it for a reason." Harry said nervously, not liking the idea of missing memories, but apprehensive about what exactly he might be missing.
"Ah, perhaps I have not made myself clear, the block in your mind exists purely in your subconscious." When that failed to garner the correct response from either Poppy or Harry, though Snape seemed to understand perfectly, Dumbledore continued. "Occasionally people will go through a traumatic experience that the mind cannot handle, when that happens a block is formed in between the conscious and subconscious mind, preventing the person from remembering. However, the block you have exists purely within your subconscious, instead of being between the two. That is only possible when the block is placed there by outside forces."
Harry's eyes widened considerably, "You mean someone obliviated me or something?"
Dumbledore seemed to ponder this for a second, "No, I don't believe it was obliviation, if what Professor Snape says of the strength of the block is accurate, which I am sure it is, then obliviation would not have been sufficient. I am sorry to have to say this, but it is likely that what you remember to have happened after the TriWizard tournament at the end of this last year is unlikely to be what truly occurred. It is the only time I can think that anyone with ill intent would have had access to you for long enough to be able to place a block on you."
Harry inhaled sharply and paled drastically. " Can it be removed?" His breaths continued to come sharply and he wasn't getting any of his color back.
Dumbledore observed him for a moment before turning to Madame Pomphrey, "I believe a calming drought is in order, if you will Poppy." This seemed to burst her own bubble of shock and after she gave Harry a brief glance she bustled off, returning moments later with a vial of light blue potion.
"Drink this, Dear." She said as she gently grasped Harry's arm to get his attention and offered him the drink. His eyes snapped to her and then to the potion vial, hesitating only a moment before taking the vial and draining it in one gulp. The effect was almost instantaneous; his breathing slowed as color returned to his cheeks and his eyelids drifted slightly shut, leaving him to observe the others with a half-lidded relaxed gaze.
When eyes settled on Dumbledore he seemed to gather himself, "Can you remove the block, Professor?" his voice slurring a tiny bit.
"Yes, Harry, we can. However, it will be quite uncomfortable to you and you may require medical attention when we are done, depending upon what was blocked. Now, with your consent, we shall remove it currently. The Calming Drought should help in keeping you from fighting the process despite its discomfort." Dumbledore peered into Harry's eyes expectantly.
Nodding slightly Harry said with a determination that should have been impossible with a double dose of Calming Drought, "Do it, Dumbledore."
Dumbledore nodded and stood. Severus stood as well taking a position next to Dumbledore with his wand drawn. "Now Harry, I am going to restrain you so that you can't break our gaze's while this is happening, it will require the utmost care and skill to remove the block without causing any damage. Do you understand?" Harry nodded, looking at Dumbledore expectantly. "Very well then." With that Dumbledore flourished his wand in a few complicated gestures and the all chairs except Harry's slid a few feet away, giving them room, and Harry stiffened in his seat – suddenly finding himself incapable of moving or even blinking. "Try to relax and allow your mind to wander, Severus and I will do the rest."
Dumbledore glanced at Snape and at his nod he turned back to Harry raised his wand while gazing into his eyes and both Professors spoke simultaneously, "Legillimens."
A/N A little shorter than I wanted and perhaps not as clear but the next chapter should both muddle and clear it some.