The real question is: "Why are you making it so easy for her to exploit you?" I don't see where your mother is all that sneaky. She doesn't have to be to take advantage of you.

Put your extra money that you don't need right away in a bank account. It will be there when you want it. Consider the money that your mother took the price you paid for a valuable lesson: Your mom is not trustworthy - now you know. This goes to her basic character. She won't change. Who knows why she's that way? I'm sure an analysis of her life, especially her childhood, would yield some idea of the reason. Make that analysis, if you like. But it really doesn't matter that much at this stage of her life. She is as she is. Don't enable this behavior. That will just make you grow bitter toward her.