At school we were taught: bottom in, tummy in, chest out, shoulders back, chin up, look ahead. It stuck with me [I'm 76] I do walk a fair bit and my posture is good.
Try wearing a normal size rucksack on your back filled with weighty things [bags of oatmeal, sugar, flour, food tins, water bottles for eg. ...not altogether!]. That should pull your shoulders back a bit!
I carry a rucksack when out for a proper walk as do many people our age that I know.
Working in someone else's home

