Hello MissCarrie What a wonderful caring granddaughter you are - your Mum is obviously very caring too. Does your Grandma belong to any Church? Maybe a visit from a Priest or Minister would help if your Grandma has any religious faith.
It sounds like your Grandma is full time carer for your Grandad - her GP practice should have a carer support worker - CSWs are used to sensitively helping people in her position. I appreciate that she is probably a proud lady - proud of her commitment of caring for her husband and possibly considers that she is a 'failure' if she admits to finding it tough.
I had a brilliant CSW when I was carer for my Mum - she knew how proud I was about caring for my Mum but managed to persuade me that I was not a 'failure' by needing a bit of 'me' time. She arranged gentle counselling for me - not a 'shrink' but someone to talk to, to talk about my feelings in total confidence, so perhaps that sort of help would be good for your Grandma? Possibly your Mum could use that sort help as well - you don't have to actually live with someone to be classed as a 'carer'.
Please don't misunderstand my use of the word 'proud' - I am sure that your Grandma considers it her 'duty' to care for her husband (marriage vows - in sickness and in health etc.) and is therefore reluctant to accept help, possibly feeling she is 'failing' in some way. She certainly isn't, but it may be difficult to convince her.
Good luck to you, your Mum and your dear Grandma - I will keep all of you in my thoughts and prayers.