Should I leave my mom alone when she's crying and says she wants to be left alone?

My mom is 76 and has always been depressed, as long as I can remember. She never sees the positive and complains about everything. My dad is 78, positive and caters to her every whim. She is hateful at times, says things a mother should never say to her child. I honestly don't know what to do anymore. She takes antidepressants periodically (which I plead with her to stick with) but she isn't honest about when and how much she takes. I've spent my adult life trying to be the "distraction" and to lighten the mood, to the detriment of my own social life. I've never married, no kids. I always feel as if I needed to lighten my dad's load. He has to live with her. It feels wrong to do anything than what I've always done. But I don't feel like I'm helping. My dad sometimes cries and it's obvious that living with her negativity is affecting his health. I've tried to talk to her calmly, when she's having a good day, but she is convinced that I'm "always on his side." I can't win. I honestly don't know what to do anymore.