Janet, I have lost many friends already and as you know, I'm about the same age as you! It is hard... the crowd just keeps growing smaller and the knowledge that your time is also coming nips at your mind as well. I really think that this is part of why some experience senile dementia; it's easy to just get depressed and give up when so many you love are gone.
I think sometimes old age is just a dirty trick.
Your Mom is lucky to have you caring so deeply for her. That really is what keeps people going and able to cope.