Classic Blunder

Seems like a lot of people base their D&D on themselves. I don’t really understand that. I always try to make characters totally different from myself to try to put my head in a different space. To me it’s a key part of escapism to try on someone else’s shoes for a bit.

People often want, and try, to do that, but then fall back into something closer to their real self.
It's very common in video game RPGs that track moral choices. For example Mass Effect, where a lot of people try to make a Renegade character and take the evil choices. But then end up choosing many of the nicer options as they keep playing. Or have to keep reminding themselves that they're intentionally playing differently.
You can do this - many of us do. However when you get to writing a solid backstory, or attempt to act as that character, it's all going to be "you" flavored on some level. Kind of like interpreting a Rorschach print.

Good therapists are security researchers for helping people recognize and fix their own vulnerabilities.

And if you're good at self-reflection, you can be both the playtester and dm.

Yes, exactly!
The moment my therapist laid out the path forward to mindfulness, it felt like I was handed a "debug mode" for my own brain. Life has been transformative ever since.
That's kind of funny in a terrible way when you consider that a lot of security research is pentesting.
Therapist: "Also you're fat"
Patient: [incoherent sobbing]
Therapist: "Ok so you're insecure about that too, try to work on that..."
it's always good to see people freak out about therapists doing their job...

Does OOP always play characters with the same backstory beats? I'm always making up new insecurities/obsessions/neuroses for my characters because I feel like I can't use the same backstory twice. (They have to have something going on, though, because well-adjusted people don't make a career out of going into trap-filled holes in the ground and fighting to the death for the inhabitants' pocket change.)

I think as a teenager I played a lot of Bards because being likeable and everyone doing what you say is kind of nice when you're an awkward disempowered kid, but nowadays I mix it up. Mostly just because playing the same character repeatedly would get kind of boring for me, and I want to explore different territory, even if it's on the level of "original the hedgehog donut steal"
If they're anything like me then each character they make is focused on a different facet of themselves or something they've dealt with. The rest bleed into it in more minor 'supporting' ways while one mental illness takes the main stage.

My main DND character is a chaotic evil Cleric of Cyric whose motivation is to become Cyric's top guy all so he can get closer to the god to kill him and usurp his power.

I'm afraid of what that would say about me...

"So what's this about your recent promotion at BlackRock again?"
You're a JRPG protagonist

One of my characters is the nicest person I can possibly imagine. I figured I could use the practice.

Another one is a teenage half-orc barbarian who's basically me at that age. It's nice to remember the good and bad of that guy.

My latest character is a robot controlled by a crew of sapient alien bees. Good luck with that one, therapist.

Let’s see… You recognize that you have been a less than great person in the past, but are wanting to become better. There is probably a specific person who has inspired you on this path. However, internally you feel pulled a lot of different ways and like you are not always consciously in control of yourself.

My first character was a barbarian who despised magic and made a point to tell every mage in existence that they are bad and should feel bad.

My second character was a grave robber who accidentally unearthed a cursed slab and refused to return it to the owner, resulting in an ancient mummy cursing him. He became a Warlock against his will and made it a point to become strong enough to kill his Patron.

My third character is a normal ass dude who got involved in a situation much bigger than himself. Currently dead set on killing the gods because they suck at governing the world. Also hates magic and refuses to use magical items that require attunement.

My fourth character is a pacifist orc who's been dead for thousands of years, resurrected by an evil necromancer to do his bidding. The necromancer botched up the ritual and the orc came back with his free will, but unable to die (much to his grievance).

I guess my therapist would tell me that I hate magic and magicians?

I mean I'd imagine they'd say that a common theme with your character's is valid misgrievances with authority, especially perceived as incompetent and evil — hence deserving of retribution for disrespecting your character's natural peace.
At the very least, a therapist would identify whether you generally perceive threats/issues as internal or external - or your PCs have ultimately external threats and want to rectify that while other people's PC (like my own) usually have internally-derived
- I appreciate the effort, but I was being sarcastic, I don't need my DnD habits psychoanalyzed lol
  I'd imagine they'd say that a common theme with your character's is valid misgrievances with authority, especially perceived as incompetent and evil
  I mean, it's DnD. Fighting evil authority/authoritarian figures, be it liches, tyrants, evil dragons or whatever, is a very common trope. Same with external factors disrupting a character's peace - it's, like, half the origin stories of any fantasy character out there.
  People can absolutely imbue parts of themselves in their characters and their backstory (I've read tons of stories of closeted trans people playing characters with a different gender from theirs, for example), but it's not an exact science. In my case, I just like playing buff dudes and ~~bullying the nerds~~ bonking the squishy casters.
  Case on point: my first two characters were neutral and chaotic evil, respectively, yet I'm literally unable to kill a fly in real life. My third character was lawful good and the fourth one is a pacifist. I just like roleplaying as different characters with differing motivations and testing out new classes in the process. Do they reflect my interests? Of course they do. Do they reflect me specifically? Hardly so, unless you squint really hard to find similarities - but I suspect people sometimes try too hard to find patterns where there isn't one.

Joke's on you, I just plagiarize my backstory from other media or roll for it randomly