My mum has been in denial since a few months after she was diagnosed. Shortly after she was diagnosed I was telling her about my best friend's mum, who had recently been placed in a care home, and she asked what was wrong with her, so I told her it was dementia. "What, like me?", so I told her that she was far, far worse than mum was and since then she's been in denial, so I don't know if it was me telling her about my friend's mother that did that.
I have this year had to get her carers going into her home three times daily as when I took her for her memory clinic in February/March she wasn't eating properly at that time, and she now accepts the carers. An ex-colleague of mine, when his mum needed carers going in and she was refusing, told her point blank that it was that, or a home, and she agreed to the carers then. I don't think that's the best way to deal with dementia patients but it worked for him.
I am lucky in that my mum isn't a wanderer (that has been my biggest fear) but she has been quite resistant to anything I put in place to help her stay in her own home but once I've pushed them through and the things are in place, she is quite happy. I find the "it's worrying me that..." approach is best for my mum, if she thinks it will make me happy she will do it. And having strangers around helps, she tends to be all sweetness and light with strangers around and reverts to the nasty old biddy when it's just her and me.
I have, on occasion, had to be quite brutal with her. For example, mum had a fall in June, the carers couldn't get in, couldn't get hold of me, and had to get the police out to break in. Then once she was up and back in her chair she stopped eating and drinking, and I had to spell out what happens to an 86-year-old who doesn't eat and drink in the heat of the summer - and I stood over her while she had something to eat and drink.
She also has quite hurtful notions, like she can't now remember my dad (he died in 1978) but thinks her mum is still alive (she died in 1990). She also at one time kept asking me who was looking after my baby. I've never had a baby!