How do I deal with my Mother's growing emotional dependence upon me?

My mother has always been very emotionally dependent upon me and has always been fearful about my well being. Even when I was in my 30's, I used to call her every evening when I got home from work to let her know I got home safely, and if I forgot she'd freak out. I could deal with this before, but now that I'm living with her and caring for her it's been really getting out of hand. When I first got here in 2010 her dementia was only in its opening stages so I could leave the house occasionally and do things in a somewhat normal manner. Lately, though, her dementia has advanced at an alarming rate, so much so that she wants me to literally be by her side every minute of the day. I don't leave the house much, but when I do, even though I have a hired companion who stays with her when I'm gone, she acts as if I were going off to war. Just now, for instance, I went to the market to pick up some things and when I got home she was in a state; she begged me to sit with her and hold her hand, she said she thought I was never going to come home and she was scared to death. When I am home she begs me to stay in her room with her all day and she doesn't seem to understand that I can't do that. If I'm not in her room, she'll ask me what room I'll be in - once I tell her she begs me to stay in that room and not move to another and if I do move to another to let her know, so she'll know exactly where I am at any given moment. Besides this, if I'm not in her room with her, she'll call my name every ten to fifteen minutes, and when I go into her room she'll say she was just checking to make sure I didn't leave the house. Then she'll beg me to stay in her room with her and the whole business begins again. There are times when she's calmer and she won't call me that often, and she spends a good part of the day sleeping, too - I feel guilty about this, but I've grown to relish the times she spends sleeping, it's like a rest for me as well. I know she needs me for comfort; a few months ago, in a moment of lucidity, she told me how afraid she was of death (she's 91) and that it was this fear that made her so uneasy every day. I want to be there to make her calm, I love to see that calm smile on her face when I'm sitting next to her holding her hand, but I feel like I'm a prisoner in my own house at times. A friend recommended I get a two-way baby monitor so that when she calls for me I can just speak into it and reassure her that I'm home and am in my room, or the living room, or whatever. I'm not sure this would work. Part of me has just resigned myself to the possibility that this is going to be the way it is until she passes. Does anybody have any advice?