You are not alone.....My Mum had a change in PD medications that resulted in a spiralling down of her health and progression of PD, which altered the course of events from then on.
When Mum was in hospital one time - she had to be moved into the corridor as she was shouting out at night....in the day her anxieties / cognition increased dramatically, and she asked when my father was going to visit her - he died 36 years ago. From my experience being in hospital and out of your own environment has a really negative impact on the elderly.
With all the difficulties and illness - PD meds wouldn't have been working as efficiently and having a urinary infection causes a lot of confusion. ( my Mum behaved quite out of character when she had one. After she moved into a nursing home - I could mostly tell when she was developing a UTI as she would start to talk rubbish and muddle things up - but often sounded sensible to people who didn't know her well.)
When my Mum was at a very low point....with PD and other health issues...and had been in hospital which caused a lot of deterioration in her emotional state....I agree with you it is horrendous and so hard to cope with.
But after she moved into a nursing home - once we got over the settling issues - (for her and me!) her quality of life improved and it was good to have help with her care and to share it with someone. I felt like I was leaving her to fend for herself - but care that is consistent (and there all the time - no waiting for carers to arrive) and becomes the new normal can be a positive thing.
The series of events that have then caused the decision made today for your Mum - cannot be reversed.(I had many sleepless nights about the decision to alter Mum's PD meds that had such a long term negative result)..but hopefully things will level out a little. The dementia and PD probably won't reverse but the affects of long time in hospital, the reduced effectiveness of PD drugs while other infections are going on, and the UTI's may lessen the difficulties you are watching now. I hope so. Your Mum being diagnosed with diabetes is not easy but with the diagnosis now - time will help the medics to stabilise her sugar levels which may help her general well-being.
I would not have believed that things could have changed for the better for my Mum but they did.
Look after yourself .....I didn't do a good job of that for myself....