Personal Horror

"It's your sense of alienation from self that provides, perhaps, the deepest terror. Where other meters measure how traumatized you are by the things that happen to you, Self measures how traumatized you are by your own reactions to those things. To put it another way, the only thing you can ever really be 100% sure of is 'I think, therefore I am.' The Self meter measures how uncertain you are about the 'I' in that statement."

— Unknown Armies corebook.

This trope is when a painful betrayal of one's self image, self-loathing, internal conflict, personal failing/flaw or guilt is Played for Drama. It can be because of what someone did, didn't do, tried and failed to do or they were just unlucky and got caught up in a bad situation. The key part of this trope is that something goes against the characters personal moral code, leading to the alienation of self.

Can lead to Heel–Face Turn or the character becoming The Atoner. Sometimes results in Bad Dreams or the character crossing the Despair Event Horizon. Some characters have an overlap with You Are What You Hate. Often results in Heroic BSoD, sometimes after running Heroic Safe Mode.

Because this trope is personal examples will be of different kind for different characters. When editing, keep in mind to explain why an example is this trope for that character.

Can cover:

Accomplice by Inaction: You didn't do something, and now you feel guilty about it.
Became Their Own Anti Thesis: You became what you used to hate.
Brainwashed and Crazy: Can lead to this when you snap out of it.
Cold Equation: Almost out of supplies? Have fun deciding who gets to live.
Dead All Along: You find out that you or someone else was Dead All Along.
Forced into Evil: You were force to do something evil.
Gollum Made Me Do It: Your other personality did it.
Heel Realization: You realize you're the bad guy.
I Should Have Been Better: You feel guilty about not doing enough/not doing everything.

The Killer in Me: Especially the amnesiac variant.
Loss of Identity: You lose what makes you you.
The Mind Is a Plaything of the Body: Shapeshifting changes your personality.
Moment of Weakness: A temporary moment that can lead to this
My God, What Have I Done?: You feel this way because of something you did.
My Greatest Failure: Can lead to this.
Organ Autonomy: Your body betrays you.
Powerful and Helpless: All of those powers you rely on and you can't do anything.
Sadistic Choice: If forced to choose between different things that betray your sense of self. Especially if taking a third option is not available.
Tomato in the Mirror: You're not who you thought you were.
What Have I Become?: You no longer recognize yourself in a new form.

Works with their own subpage:

Babylon 5

Examples:

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Anime and Manga

Neon Genesis Evangelion: all the main cast, but Shinji Ikari might be the poster boy for this trope. A person with very high moral standards, but a weak and tender heart that cannot shoulder them against the odds. He develops a bad case of self-loathing, reaching a Despair Event Horizon after the final angel and a Moral Event Horizon for himself after Dude, She's Like, in a Coma!.
Perfect Blue, along with a healthy dose of Psychological Horror and Surreal Horror. Yay, loss of identity and emotional insecurity!
Sankarea: Rea has this when she starts to succumb to her zombie nature.
Tengen Toppa Gurren Lagann: happens to the Spiral King in the prologue/parallel works.
Inuyasha goes through this after his evil side takes over.

Comic Books

The origin of Spider-Man involves this, as Uncle Ben's death is indirectly caused by Peter's irresponsibility.

Film - Animation

The Last Unicorn starts losing her sense of self when she's transformed into a human.
In Frozen Elsa freezes her sister Anna while trying to drive her away to protect her.
Toothless in How to Train Your Dragon 2 after being Brainwashed and Crazy and killing Stoick.
In The Hunchback of Notre Dame Quasimodo inadvertently leads Frollo to The Court of Miracles when he goes there to warn everyone that Frollo is coming.
In Brother Bear Kenai is first transformed into a bear. Then he finds out that the bear he killed earlier was Koda's mother.

Film - Live Action

Taxi Driver is all about the protagonist projecting his self-loathing on others, where his antagonists are just placeholders for the things he hates about himself or wants for himself.
In The Film of the Book of 1408, the overall schtick of the titular hotel room seems to be dragging guests kicking and screaming through the darkest corners of their own subconscious and beating them over the head with their own doubts, fears, and regrets. Mike Enslin discovers this to his extreme detriment.
Inception deals with the aftermath of this. In The Reveal we find out that Cobb planted the idea that her reality isn't real in his wife's head when they were stuck in a dream. This resulted in her suicide to wake up. It's unclear if she died or woke up.
In Iron Sky Renate has a Heel Realization followed by a Heel–Face Turn when she finds out who the Nazis were.
In Red Rover Kylie killed her mother with her powers, and all of Kylie's problems and life events since then have revolved around her denial of what she is. The film ends with her acceptance and owning of her powers.
In Fantastic Beasts: The Crimes of Grindelwald Leta Lestrange is dealing with this after swapping her brother for a baby that wasn't crying so she could get some sleep. The ship sunk directly afterwards and her brother died.

Literature

This is what the horror of Room 101 in Nineteen Eighty-Four is all about. Using your greatest fears to get you to betray the most important thing about yourself, to destroy your self-image by making you betray everything and anything that's left of what you valued before they arrested you.
The title character in Eden Green is a rationalist young woman infected with an alien needle symbiote that gradually takes over her body. Her intelligence is her main weapon, and her greatest fear is losing it to the symbiote's survival-obsessed instincts.
The Imperius Curse in Harry Potter can result in this. Both times when Voldemort took over many people were being mind controlled by him.
Funny Business is entirely about the guilt the main character feels as a result of misusing her Reality Warper powers.

Live Action TV

In Game of Thrones episode Hardhome Jon Snow and the rest of the Night Watch and surviving Wildlings can only watch as the army of the dead grows by the almost 100,000 Wildlings they went to save.

Tabletop Games

The Bond and Sanity mechanics in Delta Green bring this, as Agents will start slowly burning their sanity and becoming stressed as they lose relations with the external world and isolate themselves from friends and family as their lives fall apart.
Done in both the Old World of Darkness and the new one, especially the Vampire gamelines.
Unknown Armies has the Self madness meter, which is meant to address this trope specifically.

Western Animation

Justice League does this to several of the heroes in the episode "Only A Dream". Superman's nightmare has him dealing with Power Incontinence. His heat vision kills Lois Lane and he accidentally breaks Jimmy Olsen's back when attempting to give him a hug. Meanwhile Green Lantern is dreaming about losing his identity to the ring to the point where he becomes transparent, and the Flash dreams of being unable to turn off his Super Speed} and being trapped in a world where everything else is standing still.
In Justice League: Doom the plan against Green Lantern relies on this trope.

Real Life