Perhaps it's not so much that you need to cut her off, not that that's what you're thinking but some might say that on impulse. But I think maybe you need to set up boundaries that are both fair and reasonable to you but also take into account that it's not that you don't care for your mother but that it's too much clinging that she does.

Make provison for her in such a way that allows for her to be able to have access to you but within reasonable limits. Without making her feel brushed off try to make her understand that sometimes it's just more than you can handle but you do want to be there for her. I don't know what the boundaries would be but that would be something ot think about. perhaps set it up so that you talk to her consistently but on a scheduled basis and if she calls outside of those times, and you don't answer, she knows why that is. Just some ideas.