I feel the same way. That I'm not only depressed but lazy and worthless. But if I weren't so down I wouldn't be so lazy. If I could find something to get up for (besides being a mother of two small girls) I would get things accomplished, dishes wouldn't pile up, laundry would be done. It just makes me more depressed thinking about how lazy I am. I alsoam responsible for others like you are. I am a hospice nurse aid, I've been working at this health care center for 4 years. I've dealt with a lot of death and sadness, I know it doesn't help. But I love helping these people when they need it. They are alone and afraid. And though I feel alone and afraid too, I put on a smile, I hold their hands and act strong for them. Its hard pretending to be something I'm not.
What you're doing for her is amazing, don't stop! Even if its to make up for your parents. You can admit that, so don't be hard on yourself. My career has taught me such patience with my own parents and grand parents. I was so angry and unsociable with them when I was younger.
Do something for yourself. You deserve a lot, having raised four boys, had the career you did, being married (cause that's a task on its own!).