@Bogpinkbear Sure some people have circumstances which make it harder to maintain/lose weight but it’s still down to how much you want to be slim and how much effort you’re willing to put in for life.
Don’t have the treat, lower your calories a bit more and fill up on more nutrient dense food. Getting rid of a lot of the carbs (simple carbs especially) curbs hunger and lowers cravings. On good days, do any exercise you can manage, even just arms if legs hurt. Make sure you’re measuring everything. So many people say they’re eating x amount and really aren’t. The fact you say you eat 1200-1500 cals means it obviously varies a lot.
I think it’s unfair to think that slim people are naturally so and don’t have to work hard to remain that way. Some of them are regularly hungry and have to deny themselves more food to keep their weight in check. People don’t have to do that, but when others choose not to, it’s unfair to suggest they have it harder.
You can be in a wheelchair full-time and unable to move at all or expend any calories but if you only eat what your body uses, you will be slim. It’s not easy but if you were unable to get your own food and relied on someone only giving you what you needed, you would remain slim. If that doesn’t happen, you’re overeating. There’s no two ways about it. Look at the starving people around the world and show me the ones whose genetics mean they remain fat despite not eating (you wouldn’t be able to because that doesn’t happen).
And before you assume I’m thin, I’m not. I’ve been various weights, but cutting calories has worked and when I’ve tried, I’ve lost weight. When I’ve gained, this has been on me because I’ve overeaten and not taken ownership of my weight or been motivated enough to do something about it (again, nothing wrong with this but it doesn’t mean the OP is wrong about cals in/out).