As a nurse said to me, "Sometimes you have to be the bad guy."
Sooner or later you will have to get tough about something. You're going to be criticized, but there is no way around it.
I struggled for two years getting the finances away from my mother. Periodically she would break down and ask me to handle them, but as soon as I had them in order she wanted them back. Eventually, she realized she couldn't handle it and turned them over to me.
If your mother is already messing up her finances, it is probably time to "be the bad guy" and say no.
Your handling the finances and giving her an allowances seems ideal. It was a commendable solution.
You might also try a line such as - "The rich have "people" to take care of these things" or "Mom, you've done so much for me, let me do this for you. It's just one little way I can thank you."