I can relate so much to what you're going through. I hardly ever write on here, but your story just sounds so similar to mine that I just thought you might like to hear from someone who is going through the same thing. And please please don't feel bad about your sadness and even the anger, it's understandable to feel all that. I'm 23 and my mum was diagnosed 2 years ago at the age of 58. I found it so difficult going through my final year at uni, trying to concentrate on work when there was so much else going on. There was definitely more than once when I did just curl up and cry! Plus as part of my degree I had to give presentations on Alzheimer's disease, and could barely do the research without becoming an emotional wreck! But I spoke to my tutors and everyone was so helpful and understanding, maybe you could do the same thing? You might be able to postpone your exams or maybe get extra time to complete them or something. Im doing a PhD at the moment (I decided to go into Alzheimer's research, ironically I now find it helps me to cope, knowing more about what my mum is going through) so I know how much stress you are under, never mind with the added stress of coping at home.
I too feel a little alone, my friend's are amazing and always ask how things are going, but they can never really understand how things are. And it's so hard for people to say anything that helps, as they can never really tell you everything will all work out in the end, so I feel bad burdening them with all my woes! If you ever need to talk to anyone, feel free to get in contact. And please don't feel guilty about how you are feeling!
All the very best with your exams, and with everything else.