How do I talk to my mom when all she wants to do is complain and cry?

My mother is 80 and stays in the bed most of the time, due to arthritis and and a spinal disease. She can barely walk. She has a Hoveround and a wheelchair, plus all the other things available to help when she gets up.
Mom will have a "good" week - talks about what she's doing, etc. And, then she will have a "bad" week and will complain nonstop about my dad, that she doesn't have any friends, that she doesn't go anywhere (she does get out, but not much), and that I don't understand. I am going to a counselor myself to sort through this issue, plus how to deal with a bad job, and triplets who will be 7 next month. My homelife is not easy.
Mom is like a child, sometimes, and I feel like the mother, although, if I say anything that she thinks is the wrong thing to say (even my tone of voice), when she is in these bad moods, I become the enemy because I said something she didn't like. She has had depression all her life, but nothing helps her now, she says. If I make a suggestion that I think will help her, she immediately tells me why that won't work. I don't know what to do. I just stand and listen to her cry and complain, either at her house or on the phone, when she is in one of the bad moods, and I don't say much. My dad is her main caretaker, but she is constantly telling me how mean he is. I am not there, so I don't know for sure, but I think Dad is very frustrated, as I am, and Mom is taking some things out of context. I can't tell her that she is being negative, because that sets her off, too. If I suggest watching comedies on TV, she says she has watched them, and they don't help her. I feel like Stretch Armstrong (toy from the 70's) - stretched in all directions. What do you think is going on, and what can I do to help her?