The OP hasn't been on this post for a while so I hope they won't mind me giving you some of mine
Try and step into their world, they can't step into ours, don't argue about what they are telling you, it's real to them.
When people tell you things, pay attention to how they are feeling and respond to this. Acknowledge if they sound upset, scared, happy etc. Look at the VERA technique.
Even in very late stage dementia there is capacity for joy. We retain our senses. Use touch, music, taste, textures. Blow bubbles, have a hand massage, do a foot spa, try a little bit of chocolate.
Make it personal, what does the person love or hate, what did they do for work, what hobbies do they have? Look through old photos together or memorabilia. Watch clips of their favourite singer on YouTube. Cook together, if they can't do everything maybe they can stir or peel while you do the other bits.
Help them be as independent and useful as they can. Let them do things, even if it would be quicker to do it yourself, even if you'll have to redo it later. Activity is meaningful to us all, even the small things.