I'm really sorry you are feeling so disheartened. You must feel so torn, wanting to be there for him and yet it is draining you. You care too much to let him be by himself too long. But he has proven to you this year that he managed to go to the store himself (if I remember correct me if I'm wrong). I know he is failing in health though and it is worse. I know you love him. I know he can't help everything that is going on with the dementia.
If you don't take it easy on yourself you wont be there for him at all. I know that isn't what you want. I haven't heard about you doing anything for YOU. Please, think about something that would comfort you. I know you are in pain physically as well, but please find something that will give you some joy. Maybe start (if you haven't yet), putting up decorations. You can cook, maybe make something sweet and tasty. I made a marble cake yesterday and the smell was wonderful in here.
sending you good thoughts and hope you feel a little better tomorrow.