As someone with that issue, it's usually our upbringing. Parents who don't give a shit, or actively discourage the betterment of our lives.
It also happens when some people have the higher metabolism and don't gain a fucking pound no matter how much carbs and sugar they have in a day, every day, think that their offspring will be the same and then "Whoa, why the fuck are you so fat kid? You're a lazy fuck, I'll bet." when they haven't taught their kids about any of that stuff and eat like a heifer themselves while lazing about all day.
I have tried so many times to change my eating habits, but my family makes it damn near impossible between the fact that I'm the only one in the house to cook food period, the fact that my remaining grandmother has diabetes and I have to basically force her to eat even moderately well when she's super fucking picky and swears up and down we're abusing her if we don't fix her what she wants every night, the womenfolk screeching that I'm starving myself if I don't eat tons of carbs a day and having to listen to the women scream about not having sweets on hand (Though, mostly my Grandma). It's highly stressful, and I don't have it in me to cook something that I would prefer for what I would like for my Dietary plan after dancing like a monkey for the womenfolk in my house hold. I usually don't have the money either to buy my own set of groceries when everything is said and done (Bills, gas, general groceries and necessities, etc.)
I'm the only one in the house to cook food period
the womenfolk screeching
womenfolk
Why aren't they helping? Anyone can learn to cook - if they want/need to.
You sound like they have enslaved you and you can't afford to get out? Get a rent and a roommate to split costs, and maybe some peace of mind, unless you think your grandmother would be neglected.
Maybe a part time job so you spend less time in that environment? Maybe it's time to put grandma in a home or with a different relative? How much more can you endure? You are making their lives great while they seem to make yours miserable. How long have you been taking care of them? My family moved my grandmother around to live with a different family member three times, a couple of years each until she needed too much and had to go into a home.
For my mother, it's partially because my grandmother is unreasonable and expects "Restaurant quality" foods yet an extremely limited menu to only exactly what she likes, how she likes it. The other part is she's just lazy (Wants premade foods all the time, from a box or can or otherwise food she doesn't have to put any work into making).
For my grandmother, it's because she has no sense of taste, is a danger when she cooks (to both the kitchen/house, herself and us) and she just can't cook despite what she thinks. I've had to ban her from cooking for the most part because she'll try to cook everything on a flat iron because "That's all she can lift," she cooks almost always on max temp and leans over it to the point she's basically hovering over it and is likely to get splatters of hot grease/fat on her face or eyes. She cooks roasts from frozen and slathered in vegetable oil (Because she thinks it's good for you) and she keeps hounding me to DEEP FRY THINGS IN A PRESSURE COOKER! She has no sense anymore.
I can't afford to get out for various reasons. My grandmother burnt her bridges with everyone else, no one will take her, and we can't afford a care facility. If I left, her and my mother would kill each other because they've hated each other since before Grandpa died (And he died before I was born) and my mother never grew up from being a Daddy's girl. I also can't afford to leave because even studio apartments around here are upwards of 1.6K a month for rent. I also can't afford to get a job at the moment also because of my mother and grandmother. Mom would totally neglect Grandma if left alone with her, and grandma would die in a weak because she would neglect her diabetes and Mom wouldn't help because having to do anything makes mom have a tantrum like a toddler.
Doesn't help that my own medical conditions can rapidly change from "Okay-ish" to "Can barely walk and needs to be hospitalized" like I was last year around Halloween. We're all stuck with each other, and Dad's working himself into an early grave to no thanks from Mom. I can only try to help where I can and I hate myself every day for it.
God grant you the patience of Job.
What does your Dad think about the situation?