I was in a similar position to you in 2015, Sanpan, and would say that all you can do is your best in these circumstances. There are two pieces of advice that I do have: 1) Try to get your mum involved in some local activities. Mine used to love singing, and even when she couldn't find words for a conversation, she could remember those to old favourite songs. Locally, there was a 'singing for the brain' session - two hours of singing. I live 200 miles away, my brother worked full-time and his family had turned against Mum, so getting her to the group was a challenge, but the problem was solved by employing a local caring agency for 3 hours a week. That got Mum there, the carer and she both enjoyed the sessions, and she got back again. There was also time for the pair of them to have a chat and the carer to make Mum something to eat. In fact, they would have lunch together, so it became a once-a-month trip for Mum to look forward to. As the disease progressed and Mum got worse, it remained a focal point of her life - but I'm pleased we started this before she became really bad, or I wouldn't have suggested it/ she would have refused to go/ the carer wouldn't have been prepared to risk taking her.
A knock-on effect of that club was that Mum already knew a carer, so when she needed more regular care in her home, it wasn't quite such a trauma organising it.
2) Do as much with your mum as you can now. Mine died last year. The previous year, she had wanted to go on holiday with a club she'd belonged to for 20 years. The secretary phoned me to tell me she had booked, but they really didn't think they could cope with her in her condition, so I asked if it would be OK if I went along, too. That 'holiday' was difficult:: Mum was also suffering from oesophageal cancer, so finding something that she could eat was a trial (the hotel - The Clarence in Blackpool - couldn't have been more helpful!); walking was difficult for her (she was suffering from sciatica, too); she was puzzled about being in the hotel and couldn't find her way around. However, I saw her laugh more that week than I had for months or than she ever did afterwards; being among her friends was a great boost to her mood; she really enjoyed the hotel's entertainment. For me, it means I've got something to look back on.
I wish you the best of luck and hope you, too, can find things to do with your mum that will give you comfort in years to come!