If she is unwilling to allow you to handle her finances with out her lying and plain making life a living hell for you.
You can ask the administrator at the assisted living center to get you names of legit people who are bonded and trained to manage money affairs for the elderly. Your local county clerks office also should have a list. They charge either a flat fee or an hourly charge.

Then give Mom the option.

I am sorry you are having to deal with all this.