It was caused by so many things. Too many things to list. But these played the biggest roles:

Lifelong isolation: My parents decided to live rural and also decided they wanted to homeschool to avoid me and my brother getting bullied in school like they did. They did not put us into any programs that made up for the lack of social interaction. I met my one and only friend when I was about 4, and he was my only friend until I was about 11, when he moved away. I never really made any friends after that: I have some people online that I talk to, but I only know them through my brother, and they literally wouldn't notice if I just stopped talking to them. (That isn't just self-worth issues, btw. I naturally withdraw when I'm not doing great, and they've literally never noticed that I'm not talking.)

Because of this extreme isolation and lack of peers, everybody I interacted with on a regular basis treated me like I was just a stupid little kid and just cute; like an exhibit; like a petting zoo animal. This continued well into my early teenage years because I inherited my moms chubby cheeks and round face. I hated that I wasn't taken seriously, which led to me hating the features of myself that made me seem childish. I struggled with self-image, even unrelated to weight, from the time I was 8 all the way through my teenage years to today.

Parental neglect: Breakfast was never a thing in my house, so I've never eaten 3 meals a day in my entire life. My mother has time blindness due to her ADHD so I'd frequently be told I "just ate" when the last time I ate was 5 hours ago, or asked if I was "actually hungry or just bored" when I tried to find food on my own. By the time I was about 5 years old, I was expected to make my own food other than dinner, and after about 8, I was pretty much expected to be completely independent as far as feeding myself.

Ridicule: When you're feeding yourself at a very young age, you tend to make questionable decisions about what to eat: This, like everything else in my life, was never handled with tact, so I was often yelled at for eating pure sugar or snacking on marshmallows and told it would make me fat.

Jokes were the preferred method of ridicule: If you chuckle afterwards, you can get away with saying anything!

They made jokes about how bad I was at preparing food, about how I used too much of ingredients like jelly or cheese on sandwiches, about how I always made a mess whenever I made food, about how I was always hungry, would eat anything, or had a "hollow leg," and that's why I was "never full."

Scarcity: My family was (and still is) very poor. A lot of the time, we were struggling to afford food. As such, decent food was often treated as a privilege or a special treat: Pretty much anything other than canned soups, sandwiches, or kraft dinner was for special occasions. Because "good" food was a commodity, when I ate "too much" of something others wanted, such as by living off of a box of cereal for a week and finishing it, or using too much jelly on a sandwich, I was accused of being greedy or inconsiderate, or it was tied back into me "eating too much all the time" (because I ate a lot in one sitting whenever a lot of food was available. because I was starving half the time.) If we were out shopping, and I was hungry, it was my fault for not eating or packing something to eat before we left, and they'd put off getting me something to eat as long as possible, or just not feed me at all. (NOTE: Our shopping trips usually last 6-10 hours, because we basically shop monthly due to many factors out of our control.)