I think those of us who have personal experience of this will understand how you are feeling. You are entitled to feel sad, and cheated, and angry because it is a horrible cruel disease which gradually strips away who the person was as well as steal the future.
Try and enjoy what you still have together and most importantly look after yourself. Accept any help which is offered, its too hard a job to do on your own. I have just started volunteering at a day centre for people with dementia. Everyone who goes is happy to be there and I am happy to spend time there, knowing that carers can enjoy a day to do something, or do nothing. We all need a break sometimes.
My lovely mum died 3 years ago after a long hard l0 years with alzheimers. It was the hardest time, especially as she didnt know who i was for the last 7.
Many of us have been touched by this disease and though so very hard I believe it makes us better people, more understanding to those confused people we come across from time to time. Certainly recently in a supermarket a lady who was with her daughter wanted to dance with me at the checkout, and dance with her i did. I understood just how her daughter felt.
Alzheimers Society have a lot of helpful information and support. I phoned them, just to talk, more than once. Good luck to you and everyone in this awful situation.