Hi
@Sue T Heb and welcome to Dementia Support Forum our friendly and helpful community of people who have experience of many aspects of dementia.
I am sorry to hear about your mum. People with dementia often say the cruellest things to those closest to them. It may be a sign of their frustration at something indefinable being wrong that they inevitably blame on others. Because people with dementia start to lose empathy and awareness of their own limitations, it is often the closest family members who they target for the most unthinking and hurtful remarks.
This is something that I and many others here have learned the hard way. I know how difficult it is not to react when nasty things are said. All I can suggest is that you try to distinguish between the mum whose personality still lives on inside and the dementia that has changed and now controls how she thinks, acts and speaks.
Try not to let yourself become a victim of dementia. I am certain that what she has said is not an expression of how your mum really feels. And if you find it hard to cope, come back here for understanding, empathy and support.