Oh my goodness, your mum is looking down on you and she is so incredibly proud. You are amazing, doing all this for your grandad. And she will be absolutely raging with your family and how little they are doing to help you.
I really hope you understand what an amazing thing you are doing for him, and also I hope you understand that you have been absolutely let down by every other member of your family (apart from your dad). They should be throughly ashamed of themselves. No matter what happens from here on in you have absolutely nothing to be ashamed of, and absolutely no reason to feel guilty or feel you have not done enough.
I am in my 40s, married to a lovely supportive husband, and we are really struggling to get proper care for his parents. So it must feel a hundred times harder for you on your own and being so young.
Sadly I think it often takes things reaching crisis point for anything to happen. My mil is in hospital and I have refused to take responsibility for her care when she leaves. I feel guilty as anything that she's stuck in hospital waiting for a care package, but we are only being taken seriously now because she's taking up a hospital bed.
We're in Scotland and I found the helpline Alzheimers Scotland to be hugely helpful for talking about a family member with dementia. Their advice line is staffed by volunteers who have all been carers so they totally get it. I'm sure there will be similar in England, or just phone the scottish one. How will they know where you live? I have cried to those people so many times. Call them.
I'm sorry I don't have any other advice. If you don't feel that you can just walk away (I would find that really hard too) I think you have to stop looking to the family for support. You are not going to get any there so stop wasting your energy trying. I know it's rubbish, but that's life. They are shitty, selfish people.
Focus your efforts on social services, gp surgery, alzheimer's support groups. Are there any local groups you could get him to? Our local church runs a weekly music group. Or would he not go, is it too hard to get him out? Sometimes just making a connection with other people who get it can make all the difference.
I am not making excuses for your family, they don't deserve any. But if they don't see him day to day like you do they might well honestly not realise how bad he is or how much care he needs. Could you say 'I need to go away for 3 days next week, you will need to look after grandad' and then go? Maybe that would give them a shock to realise what is involved. Or maybe they would just do the bare minimum and you will have to pick up the pieces.
I'm sorry this has all landed on you. It's not fair.