How do you tell your long distance BFF that you're no longer interested in being friends?

I have been friends with this person for 30 years. We live in different states. On the most serious times of my life (husband hospitalized several times ill) she didn't even call. When I do speak to her it is always about her plus she is cheating on her husband and I just don't agree with what she is doing and have told her so. She told me once she became a minister but I doubt highly that ever happened. She is not in good health either so there is a guilt factor. I just can't take all the BS with her anymore. I tried to just stop contacting her but a few months will go by and I will get a sappy email from her. It would be hard enough to have a conversation with her and just spit it out but I want to be as gentle as possible. Any advise?