This relationship does sound unhealthy.

I don’t know if a letter of explanation is needed, and will likely backfire. People get angry and lash out when confronted.

I hate to suggest you be passive aggressive about severing ties with her, but that’s what I’d try to do. Just be too busy to have any conversation with her longer than fifteen minutes. Twelve hour conversations are unheard of. It sounds like this person is trying to dominate, torture, and brainwash you.