Elle, glad to finally hear your story.
I can really empathize with both of you. One, by introducing bff into the circle it just becomes inevitible, really, that you're going to develop feelings. At least it seems that way to me.
The other is with your husband. If he wants sex daily and you're just going through the motions, it's really shocking to hear that you'd rather get it somewhere else. I'm only saying this from what I presume is a self-centered position on his part. I'd love my wife to have a libido that could go at least once a day, but not at the expense of her pleasure. So from your perspective, I totally get that just because he wants it, doesn't mean your needs are met. But do try to understand his perspective. Have you all seen a sex therapist as well as a counselor?
Me: 40 straight male in a V with
Wife: Kay - mono female - married 18 years
LDR girlfriend: Susan - poly female - 3 year relationship
Stakes - very intimate friend
Ironically named Farmgirl - local interest