I have every sympathy @Anneishere - I’ve been through all this twice, with my mother and my FiL.
First, can I please advise you strongly to join the Talking Point forum on the Alzheimer’s Society website. The carers’ forum was a lifeline for me. There aren’t often any easy answers but at least you can offload to people who know exactly what it’s like - people who haven’t lived with it hardly ever do. (But they often think they do and like to give unsolicited and almost certainly useless ‘advice’.).
Being horrible to someone close to them is quite common among people with dementia. It’s all too easy to say, ‘Ignore it - it’s not them talking, it’s the dementia’ but it can be so hard not to feel very hurt by nasty comments or accusations of stealing, etc. My own usually incredibly robust brother was once reduced to tears by the vile things my mother had said to him - and he’d been the Golden Boy who could do no wrong.,
One thing I would advise - and it took me and dh quite a while to suss it out when we were very new to dementia, no internet help then! - is not to try to correct the person when they come out with something you absolutely know can’t be true. Just go along with it as far as humanly possible.
As an example, for quite a while my mother was convinced that her sister had ‘stolen’ their mother’s house. In the beginning I did try to reason with her and explain the facts, but she simply wouldn’t have it and would furiously accuse me of being ‘in league with’ her sister (my aunt.)
Eventually (it cropped up again and again, and she never could remember any previous conversation) I learned to say e.g., ‘Dear me, that’s terrible, I had no idea, I’ll get on to the police/a solicitor first thing tomorrow.’
That always kept her happy, or at least reasonably contented, for the moment.
Lastly, please do not allow yourself to be guilt-tripped into doing more than you feel well able to. Dementia is only ever going to get worse, and as anyone who’s lived with it will know, it can be incredibly stressful and exhausting to cope with. If social services are, or will be, involved, do make your boundaries crystal clear - it’s so much cheaper and easier for them to offload necessary care on to relatives.
All the best. ?