Have you thought about Abbeyfield's sheltered housing. It's usually an adapted big house housing for about 9 residents. They have them in most towns. It's run by a charity. Mum and Dad were there for a bit. The house manager was fantastic. They had regular outings and socials arranged. They had meals provided. They could have carers in if they were needed. It's rented accomodation but I thought it was very reasonable. Dad didn't like it though but that's another story
Jenny,
I do agree with Crocus that you need to see your GP before making any decisions. When I was trying to decide whether to move closer to my mum, I saw a counsellor who was very helpful as she could challenge some of the assumptions I had made about what would or wouldn't work. And I too am on ADs which help me to function and hopefully think a bit clearer.
I would imagine a house with granny flat would always sell very well - either for relatives or au pairs etc - so you would get your money back. The most important thing is though how would your MIL feel about it? Would she be expecting you to be at her beck and call being next door? Would she even agree to it? Potentially you would build it and then she could refuse to go! And surely if she sold her house, that would more than cover the cost of granny flat in any case.
I do think GP / counsellor is the way to go first though. Many of us (myself included) feel that our life as we knew it and wanted it is on hold / over. I personally love travel but now cannot do it for more than a couple of days at a time. I long to go back-packing around Asia again but alas ..... It is the price we pay for caring. For most of us the price is worth it; for some, it is not.
Incidentally, the middle-aged woman you describe in the supermarket dragging an elderly relative around is probably me but this does not mean I am "docile / defeated / Stepford woman". Quite the contrary. It is just my priority in life now has to be my mum. It is not her fault (nor your MIL); they would probably be horrified if they saw the sacrifices we are making. It is why this Forum is so good - people understand those feelings.