I think it's quite common for the deterioration to become suddenly apparent when the person is out of their home environment, and it sounds as if the time has come to organise some additional daily care. My mother was also self funding and over the course of 18 months I increased her care at home from four hours a day to six hours a day. The 'care' was to do practical things but also just to keep her company as she was anxious when left alone.
I found it best not to discuss the options with her, I just went ahead and did it. When presenting any changes I put on my game face and was cheerful and positive about it - if I showed I was upset or uncertain it would immediately upset her, she needed reassurance everything was 'okay'. I never mentioned the word carer, I just said a nice lady was coming to help her. When the time came for a care home, again I didn't tell her in advance because it would have made her very distressed and anxious, I organised it and told her she was going on a short break. After a few weeks settling in she told me she loved it, and 9 months later it's genuinely her new home and she's happy there.
And yes I agree - social services drop you like hot brick when they know you are self funding, you have to do it all yourself. However that does at least mean you get the choice of what type of care you provide, and what care home you choose.