Welcome to the forum. I'm sorry to hear that your Mum's talk with a Parkinson's nurse specialist did not help. I'm a lot younger than your Mum, I was diagnosed 10 years ago at age 39 but some aspects of my experience may help.
Depression is common in people with Parkinson's but I would strongly expect that the change you see in your Mum's personality or mood is a result of the diagnosis itself and not the underlying condition. The news that you have a progressive and incurable condition is enough to shake your confidence and many of us go through a sort of grieving process as a result.
Depression is common but that doesn't mean there is nothing that can be done about it. Staying positive and active is important. Are you in touch with a local Parkinson's group? It can really help to talk to other people in the same situation. That applies equally to you and your Mum.
The Parkinson's UK helpline is certainly worth a call. (their number is at the top of the page) The advisers there can help you find a local group and can help you understand any other options to get help for you both.
I hope that helps