I'm so sorry, it really is shit. My ex has young onset Alzheimers, hard to say at what age it started as it was undiagnosed for many years, but I reckon around 52-53.
We were together for 25 years and had two young children. She was always easy to love, difficult to live with, but her behaviour spiralled and it felt very abusive. Looking back, I reckon she was scared and confused and masking like mad, always lashing out and denying things we had agreed, and insisting I was an impossible bully. I was totally gaslit. She retreated into herself, got fired from her job, gave up her friendships. I worked full time, did all the childcare and housework and became enormously resentful of her.
We did try relationship counselling but it was useless - she could never retain why I was unhappy and this felt to me as her not caring enough to listen. Eventually everything crashed in - I developed severe depression, had to quit my job, couldn't pay the mortgage. The children were getting really affected and so I finished the relationship and sold the house. With the proceeds I bought two small flats in the same street, so the kids could retain both parents.
Without my support she spiralled fast. I began to suspect dementia but it took a long time to get her diagnosed. Since then - 5 years now - I have been her carer. (Not quite the fresh start I was hoping for!) It has been very hard, and there is very little available for this small group.
It has been the saddest thing watching her decline, and exhausting supporting her and our kids through it.
Thank you for the opportunity to get it off my chest...