I've spent the last 8 years watching my Grandad go through the same thing. My Mum died a few years ago so I'm the closest family he has nearby, so I see him in his care home twice a week or so and until recently we'd bring him to ours once a fortnight or so.
The only real answer is that you compartmentalise. Yes, it can be very sad when I see him, but then I go home and live and enjoy my life, and try not to think and worry about him too much. Which sounds callous I know, but worrying about him isn't actually helping the situation at all, so its pointless.
And it's not all negative. For the first few years after his diagnosis he was mostly fine, yes he was losing details and then he started not being safe at home, but he was still there, and I could have a good chat with him, and he'd offer advice about a parenting issue or some DIY I needed doing. He was still Grandad, even if I had to remind him that my daughter was a teenager and not 4 any more etc.
And even now, sometimes he's still him, even if its just in fits a starts. Is it all very circular, yes, is it upsetting when he forgets who I am? Of course it is, but if I can get him reminiscing about something, or telling jokes that I've heard a million times, then it's still comforting.
The worst bits are when he's scared or really really confused. But give it half an hour and he won't remember being scared and confused, so I try not let let it bother me too much.
In short, yes it's shit. But it's not as shit as you're probably expecting it to be right now.