Yes folks, today I want to explain why Snape probably was an Obscurial in secret. What makes me think it was the case? Well...
First, Snape has many elements that would prompt him to develop an Obscurus or something of the like.
His father hated magic and abused Severus and his mother (OotP, DH). Severus could have resented his own magic if he saw it as the reason his father was so violent. Perhaps he was well on the path of developping an Obscurus that would end up killing him, as he desired to be loved by his father and hated in turn his own magic; up until he met Lily at an early age, during which he rediscovered the wonders of magic, decided to love Lily and despise his father instead (even though inside he could still desire his love). Loving Lily and her magic could have prompted him to accept his own magic, before his Obscurus truly went out of control.
Traumatic school experience. As we all know, Severus was horribly bullied throughout school. He could have come to hate himself for being a wizard and thus having no choice but to study at Hogwarts instead of another school where things probably would have been better (where he wouldn't be tormented and humiliated on the daily by his classmates). He could have resented his magic for being so weak. He could have resented having so little control over himself (and in general, in own life), notably after losing Lily, and that urge to regain control of himself could have prompted him to try and tame his magic excessively. Besides, traumatic and violent life conditions seems to make it easier to develop an Obscurus.
He lived through war, witnessed horrible things and Lily died; again, he could have resented being a wizard, having been born at all. We know he certainly wanted to die.
In summary: His life sucked.
He’s a Master Occlumens. Occlumency has a lot to do with controlling yourself, your magic, your thoughts and emotions, which prompts me to think that excessive Occlumency could be used to keep check on an Obscurus, but also be the cause of it.
The parallel with Ariana
As you might know, Ariana was described as an Obscurial, regardless she wasn’t precisely named as such:
She wouldn’t use magic, but she couldn’t get rid of it; it turned inward and drove her mad, it exploded out of her when she couldn’t control it, and at times she was strange and dangerous.
What I couldn’t get rid of my head is this:
When my sister was six years old, she was attacked, set upon, by three Muggle boys. [...] They forced their way through the hedge, and when she couldn’t show them the trick, they got a bit carried away trying to stop the little freak doing it.
Now the parallel I talk about?
Lupin looked sideways at Sirius and then said, "Look, Harry, what you’ve got to understand is that your father and Sirius were the best in the school at whatever they did — everyone thought they were the height of cool — if they sometimes got a bit carried away —"
This is how Lupin justifies SWM, the Werewolf Incident, and the years of unrelentless torture from four wizard boys to an innocent. "They got a bit carried away."
What happened last time a similar thing happened?
It destroyed her, what they did; She was never right again.
Dumbledore about Severus:
"But I forgot — another old man’s mistake — that some wounds run too deep for the healing."
(Just like wounds caused by Dark Magic can almost never heal.)
Severus and Ariana are very alike: they both were irremedially wounded, destroyed and changed through the pain inflicted by their respective assaulters.
if Ariana can become an Obscurus from that one traumatic experience with three Muggle boys, Severus can develop one as well from countless traumatic experiences with four Wizard boys + a Muggle father. As well as whatever shit happened in his life later on.
EDIT: The parallel with Credence.
In Pottermore, we learn that Snape was whipped by his father. In FB1, Credence is whipped/betled by his... "caretaker". Credence ends up killing her, so I wonder what Severus could have made of his father.
Also it seems that Credence's love for Nagini helped him keep control over his magic, so it could parallel Snape's love for Lily that may have helped him control his Obscurus as well.
Severus is not just the best Occlumens of Wizarding Britain—and perhaps, the world—he also show signs that his magic is quite overpowered and explosive.
When Snape uses the Disarming Spell against Lockhart, he actually blasts Lockhart off his feet against the wall, which is not what Expelliarmus is supposed to do. We know that because in DH Harry wants to use Expelliarmus so it'll only disarm the opponent, whereas Stupegy would blast them off their broomsticks and kill them with the hundreds-of-meters high fall. (But if Snape had used Expelliarmus against them, who knows, they might have been blasted off their broomsticks like Lockhart was in CoS, so there's definitely a difference in Harry's and Severus' magic.)
When Severus uses Avada Kedavra against Dumbledore, which is supposed to just sweep the soul away, he blasts Dumbledore and makes him topple over the tower’s edge to his fall.
When Snape uses non-verbal Incarcerous against Lupin in the Shack: "BANG! Thin, snakelike cords burst from the end of Snape’s wand and twisted themselves around Lupin’s mouth, wrists, and ankles; he overbalanced and fell to the floor, unable to move."
When Harry tries to use Levicorpus on Snape? '"No, Potter!” screamed Snape. There was a loud BANG and Harry was soaring backward, hitting the ground hard again, and this time his wand flew out of his hand.'
When Severus shoves Harry away from his mind, his magic apparently also shoves him away physically: '"ENOUGH!" Harry felt as though he had been pushed hard in the chest; he took several staggering steps backward, hit some of the shelves covering Snape’s walls and heard something crack. Snape was shaking slightly, very white in the face.'
EDIT: as @derschokoladenritter finely pointed out in the comments, "In Chamber of Secrets, Snape cancels a whole room of spells with one Finiter Incantatem during the Dueling Club", demonstrating yet again his overpowered magic.
Perhaps this is just a result of accidental magic, but still, remember when Petunia made a jibd at Severus’ clothes being his mother’s and a branch fell from the tree, falling on Petubitch? Perhaps his Obscurus acted out of his control.
Another instance of Severus’ poor control on his own magic too: "A girl was laughing as a scrawny boy tried to mount a bucking broomstick —"
Severus learned to fly unsupported, perhaps thanks to having mastered his Obscurus and its power.
So now we got many in-books clues; but here comes the cherry:
Fantastic Beasts Quote
Ok I know it’s not in-books and all the problems that come with FB, but still, hear me out.
Why did Grindelwald search for an Obscurial? If you look at this scene, Dumbledore says:
"He had a vision, he said, many years ago, in which an Obscurial killed the man he fears above all others."
"You", Newt will say to Dumbledore.
But we know Credence isn't the one because... Snape fills that role already. Snape is the Obscurial who killed Dumbledore.
Now you'll tell me that not only is this scene out of the books and in a FB movie, which makes it credibility shaky at best, but also that this comes from a deleted scene. True. But does it make it irrevocably irrelevant? There is another deleted scene from FB1 where we are shown a dragon with three heads in Newt's magical zoo. Later, in FB2, Newt uses a description of this dragon with Tina. Meaning that the creators can still consider deleted scenes as valid, at least story-wise.
And this vision makes so much more sense than whatever bullshit they gave us at the end of FB2. Now I hope that, besides a reveal of Tobias Snape as a WW2 soldier, we get the confirmation in FB3 that Severus is the Obscurial Grindelwald was truly searching for. It was not the little girl from FB1. It was not Credence. It was Severus.
It explains why Dumbledore believes (regardless he admits it to himself or not) in visions and prophecies, even though he doesn't like Divination and wanted to stop the subject being taught at Hogwarts.
Who knows, perhaps it explains why Dumbledore treated Snape like shit. Did he know or feel that Severus was the Obscurial that would kill him? Did Dumbledore... fear him? Did he resent Severus, realizing that Grindelwald wasted his time on the wrong Obscurial? Maybe that's why Dumbledore mistreated and neglected chlid-Severus so much, as if he was already the filthiest of criminals. Only, he didn't know the vision would sort itself out in agreed suicide rather than murder. Oh, the irony Dumbledore must have felt when he realized that indeed, Severus would be the Obscurial that "killed" him, and that Dumbledore actually wanted for Snape to do it. That he asked for such a "great favour" from what Grindelwald would call "a miracle".
Severus Snape, the Obscurial Occlumens.
I would have given proper credits to the person who informed me about the deleted FB scenes, but since they don't want, the least I can do is publish as they wanted and ask you what you think about this :)