Aw, how sad. I live alone after divorce since the kids left home, but I love Sundays. I lie in bed listening to the radio till the start of The Archers, then get up and have a nice breakfast while I listen to that. I potter around doing weekly chores - washing, changing the bed, unload and load the dishwasher, pausing for cups of tea with a biscuit or a slice of cake, and watch a TV film in the afternoon. If it’s a nice day, a long walk blows the cobwebs away. I always cook something interesting for supper, then prepare my stuff for Monday morning before I go to bed.
Instead of getting wistful about what you don’t have, make it into a day of things you do have. Try to fill the time on Sundays with things that make you feel good, doing things you enjoy and being a little self-indulgent.