I know how you feel. I was the sounding board for many of my mother's anxieties and she took a lot of her frustrations out on me. At first I thought it was 'just' Mum's need to express negative thoughts as she coped with her PD, but as time went on I learned it was all part of PD and 'out of her control.' That did make it a bit easier to bear when things were stressful.
My Mum's doctor told me that Mum needed to hear when things got hard for me or her behaviour was causing me difficulties, which made it easier for me to say 'No' I can't do that now - but will do it later...when it wasn't an urgent task.
You are not alone and finding someone to unload with about things is a really good idea. As time went on I got better at not letting Mum's anxious and stressful conversations 'get to me' by accepting what she said to me and telling her i'd deal with it.eg: she'd go on about how something had gone wrong and she wanted me to do X to put it right....initially I'd try to problem solve or discuss with her that X didn't need to be done straight away......later on I'd say to her "thanks for letting me know, leave it with me and I'll sort it out"
Look after yourself, a carer's job is a tough one at times.....