Re-assure your Mum there is plenty of good times ahead with fewer responsibilities. Eat well, go out, socialise, exercise, pray, take the tablets and laugh a lot.
@AintNoFlorenceNightingale Good advice above. Get your mum to go to dementia groups, cafes and craft sessions. It may not have been her thing prior to dementia but these groups give support, understanding, and the opportunity to make friends with people in the same situation. Her new friends will understand her fears and will show her there is life after a dementia diagnosis.
I have made friends with two couples whose husband's have dementia. we go to the cinema , musicals and concerts and the other couple is off on a coach holiday together. My OH not up to the journey or we would be going too. X