So sorry to hear about your mums problems and welcome to the forum.
I take it you are her carer so you may well find lots of help and info in the carers section,
please don't think your being selfish in finding it hard to come to terms with your mums dementia dx it's hard enough on you dealing with pd let alone having that as well.
All careres do a wonderful job and we pd sufferers would be so much worse of without you all,don't forget you have to look after you your self as well as your mum so any chainge for her is also a change for you, you need time and support to cope just as much as your mum. If not more so as your the one who has to cope with all the changes , and please don't forget to treat your self kindly as you have a life to live aswel .
Hope things don't get to bad to quickly and do keep coming on here. There's loads on here who will cheer you up. Listen to you Give advise if thay can n support when you need it.
Take care n live well. Cc