my 82 year old mother lives in an apt attached to my house. Most of her day is spent in the house alone. I feel quilty (and sorry for her) all the time. I work full time and have a teenage boy to raise. I always feel that I should be spending more time with her, but I am overwhelmed at all the things I have to do. I just feel horrible and have the image of her all alone in the bedroom all the time.
How does one shake these feelings?

It is difficult, but, how about finding a younger older person who has transportation and see if you can get a friendship going. Visits from another elderly person just once a week does wonders for people like your mom.

When you are home, include her in as much activity as possible so she does not feel like she is just there but is actually a part of the family.

Sometimes, elderly people are able to do small things in the home, maybe straightening up the magazines on coffee table, dust a little, etc. and it makes them feel more a part of the family. No, don't make a slave out of her but the elderly want to be needed.