What does a caregiver do when they are about to lose it?

I know the Christmas season and all is a stressful time for everyone. My stress is a buildup of caregiving for 3 years all by myself with NO relief and feeling guilty for any little break that I may take away from the house, like the grocery store, etc. cause that is all it has been..........My Mom is turning spiteful. She sets and pees in her Depends and when I ask if she is peeing, she says "well, I need to go." She is as able to get up and go as I am!........After a bout with pneumonia a month and a half ago, she used me and played me by having to lift, tug and carry her to the bathroom and bed, until I almost got down myself. My husband and daughter watched her and when I wasn't around close or in another room, she did it herself. I mean I was carrying, lifting and the works, on the pot, wiping her, changing her Depends and everything..........So now she is doing it in her Depends and she knows she is and then she ignores me when I tell her she needs to go change them. I told her today, oh, there's no need, you've already went and she just set back in her chair and started rocking. If she has to go while she is eating, she just sets up straight in her chair and pees herself!! She has it written across her face and I can tell from her actions.........If my eyes even look like they are looking for something, "what are you looking for?", umpteen times a day........I put on my coat to go feed our animals "where are you going?", umpteen times a day, even going outside for something.........My daughter needed something, can't remember what is was, and it usually lays on the bar that is our room divider. My Mom said to her "what are you looking for?" My daughter told her and she got up and took her into another room and showed her!!!! Not where me or my husband or daughter had put it or left it. I'm guessing where she put it!!! Me and my husband are always accusing each other of losing stuff and it's not us, we find out. It's a wonder we haven't had a big blow up over this and she is the one doing it. My husband has been so supportive of me taking care of my mom, but now it is affecting me and my mental and physical help. Does anyone have any helpful hints or ideas of what to do when you are just a smidgeon from losing it?? Please help!!