Hi @Lemonjuice I have cared for my husband for more than 15 years, and throughout that time as he gets diagnosed with more medical conditions ( two are untreatable), I have tried different strategies at different times to get him ready for a taxi trip to either doctor, specialist or hospital. These are only suggestions. My husband and I have been together 30 years and he is a narcissist and bully and there is a history of psychological and verbal abuse, but as his health continues to deteriorate,he is reliant on me, which annoys the heck out of him. I have my own mental health and trauma issues, so I possibly understand how difficult your situation with your mum might be.
There were times early on when I asked hubby nicely to shower and dress to prepare for said appointment. That never worked. I would ask hubby if he maybe thought he needed to go into hospital, always gave him an option. That never worked because he hates hopital so the answer was always no. @Lemonjuice, it is important for you that you look after yourself first, because caring for someone is tough work. What works for me now is I don't give hubby a choice. I tell him what is going to happen: he is going to x y z and I am getting him into the shower and dressed into the clean clothes I've put out for him, and we are ringing a taxi. And I am like a sheep dog..I nag him and follow him around and just keep him moving. Once he's in the shower he realises I am like a dog with a bone...I will not stop until we are on our way. He admits it is much easier for him to go with the flow and let me have my way. And in our case, this tough love works.
Wishing you all the best. Take care. 
