P.M.
I hear your confusion and heartache, and hope you will find your way to a solution. As an older woman, I have experienced that our ability to handle stress does tend to decline with age, sometimes precipitously, and that overwhelming stress can have a major impact on outlook and mood. Add in the hormonal changes that come with perimenopause (which can occur over 10-15 years in some women), and the complications of adding more people with their own issues to your household, and I'm guessing your mother might often feel at the end of her emotional rope.
Of course, there's also the possibility that your mom's mental state is something more than all this, and could include some age-related deterioration or mental illness of her own.
In either case, communication is critical to help all of you assess how to address current problems, and where to go next.
I hope you'll check out a practical and effective communication technique called Non-Violent Communication. You can learn a strategy that will help you talk to your mom compassionately and respectfully, so that you don't add to her stress. You can help her identify and understand her own needs, some of which are apparently going unmet. You can learn how to make clear and unbiased observations that will support, rather than undermine, further communication. And you can state your needs gently and clearly, so that the other person can hear them without feeling attacked or overwhelmed.
Non-Violent Communication is a win-win process, and can be effective even when used by only one person in a relationship. If this interests you, you can google the term for books, classes, summaries and examples. My best to you all.