Anxiety can be caused by lots of different stuff. Genetics, traumatic experiences, hormonal responses in the brain, generational trauma, but often it's developed at a young age as a defense mechanism. You can't survive on your own as a kid. You're dependent on the people around you to keep you safe. So, if those people are neglectful, or abusive (either emotionally or physically), then your poor baby brain is going to try to compensate.
It wants you to survive. It wants you to feel safe. So, it needs to figure out why you're not safe and how to fix it.
"Why did dad leave?"
"Why did mom yell at me?"
"Maybe I did something bad."
"Maybe I'm bad."
As sad as it is, that's the only way that a kid's brain can make sense of a hard situation. A kid can't say to themselves, "Maybe dad's anger has nothing to do with me. Or maybe mom's neglect is because she's overwhelmed." Nope. All your brain can think is, "My parents aren't taking care of me. It must be my fault, and I better fix it or I won't survive." But your brain doesn't know how to fix it. It's the brain of a baby. So, it hijacks your stress response system and acts like an asshole to keep you in your place. "Stop making a mess, you idiot. What if mom sees? She'll probably go get puppet cigarettes and never come back". In a twisted way, it thinks it's protecting you.
The more your stress response activates, the stronger it becomes. Which means you can be a full ass adult, but your baby brain has gotten so entrenched that it's applying your stress response to all sorts of things that aren't actually dangerous. So, your brain hijacked your stress response system when you were young to keep you safe, and it got stuck there. So, even though you're grown, you're still trying to function with a system you developed when you were a baby. It's kind of like your computer running a very, very old operating system, which means lots of critical errors, lots of glitches, and lots of pain.
But there is good news. Your amygdala is what sciency people call plastic. Basically, it can change. And if it can change, that means your stress response can change, which means your anxiety can fly away. You just have to apply the right techniques.
First of all, you have to try to be kind and patient to yourself. I know, I know, I know. We hate hearing that, but it's true. When you get mad at your brain, you're basically getting mad at a scared kid. a kid that didn't have the support they needed when they were young. And so we have to be the loving parent that they didn't have.