Lucky If I've understood correctly, this lady has managed without your shoulder for the twenty years that you were out of her area? How did she manage without you? Did she latch on to someone else?
This may sound harsh, but I do speak from experience here! I was the twenty-five year shoulder for someone who phoned practically every day and would talk/moan for an hour or two. This was before cordless phones or caller id. I would probably have answered anyway because I would have felt so guilty if anything had happened to said person. That's what traps you, the feelings of guilt!!
When my circumstances changed (ill health) and I was no longer 'on tap', she immediately filled the gap that I left with someone else. So, what I am saying is, don't feel bad about putting her off, she will, in all probability ,find someone else. Unfortunate for them, but it would be their decision whether, or not, to become involved.
Hope you manage to resolve this without feeling bad about it. My conscience pricked me for ages until I discovered that I'd been replaced!