How do I handle my angry and aggressive mother without getting sad or depressed?

My mother is is assisted living. Mostly she is in her wheelchair. Today, I was dong some chores in her room and she was wheeling herself down the hall. I went out to look for her and she was inside a spare room which was full of the belongings of another resident. The room door was opened and she went right in. She was holding a small jewelry box and trying to open it. I told her she needed to put it back as someone could think she ws trying to steal something. She became furious and I literally had to wrest the box from her hands. At that point she called me a bit**. Took her back to her apartment. She was furious and told me she could do whatever she wanted. Would not listen to me at all when I tried to explain why this was inappropriate. She went on to say a number of rude things. eventually i just left.
Mother has always been a bit of a diva and had wicked tongue when she was thwarted, buyt this behavior just threw me for a loop. She is on mes for mild dementia and I think she is having TIAs quite often. Just at a loss as to how to deal with her. She refuses to cooperate with the attendants and I fear she will have to go to a nursing home soon. She absolutely will NOT listen to me or to any of mysiblings, doctors or anyone else. She seems determined to do what she wants and expects everyone to accept it. How to handle this w/o getting angry, sad, depressed, etc? I feel totally helpless.