Hi and welcome!
Like a spice to soup, you "added" her!
Okay, I'm assuming D is a woman. Is she a platonic friend with you or a lover to both you and K?
We are definitely not lovers. In fact, we have a hard time trying to have any level of friendship. Civility has been difficult. She has said that if it weren't for K, she probably wouldn't keep my company or be friends with me at all. I try to keep my mind and heart open to her, listen, try to understand her various struggles, and express mine. She can't hear me very well, though, and seems like it is a real hardship and sacrifice to her to consider my and K's relationship. So communication is quite bad. No matter how I approach it she usually becomes very defensive-turned-offensive about it.
I find it interesting that she was never comfortable with polyamory yet has stayed in the relationship for two years and even moved in! Do you know what her issues are? You say "we" are struggling, so I take that to mean all of you are, not just D. What is the focus of the struggle?
The focus of the struggle for all of us, first, is the lack of communication skills and willingness to communicate respectfully. I feel that I am willing, I try and start from a place of compassion but lose it when I am disrespected and told various things (such as when I refer to K as my partner, D, will snort and say that because of some of my current temporary life situations)* K and I don't qualify as having a partnership in her opinion.
She moved in as K's friend in need of a place to live after leaving her husband of 38 years. She lived here for about six months and we all tried to be friends, and more or less succeeded. She clearly showed some growth and ability to work on her issues after leaving her ex, and showed interest in K. Eventually, K and I agreed that we could invite her to share our bed, and she if she was interested. She definitely was, (except that she was truly interested in K's bed) and our crazy ride started. She almost immediately became firmly attached to K's hip, and began resenting any exclusive time K and I had. K likes to believe in the good of people and we both realized that the first experience of poly can take a lot of adjustment time, so we kept trying to work it and work with her. He kept thinking it would resolve to either her becoming okay and comfortable with poly or that she would realize that it was not for her and that she would leave. And of course, he fell in love with her, and he really hoped she'd get it. Well, it's over two years later, ad she's still having problems with it, and I am having problems with the way she treats me; K is totally frustrated and just wants me and her to "get our sh*t figured out," his communication around it has been awful. I'm at the point that I have tried to make peace, reach out, volunteered to compromise some of my hopes and needs, and she always wants more. I think I might be coming to the end of this for myself. It's unhealthy, I see no movement towards change, I seem unable to effect change. I am losing some of my trust in K around communication; and I feel he should be more protective of our relationship, though I know he tells D that it doesn't matter what she thinks, I am staying here as long as I want, and it's up to us to decide what our relationship is. I am starting to wonder about his boundaries in continuing in relationship with someone so ill adapted to poly for so long.
Due to my situation (see * below), I really have little for options, and my heart breaks when I think about leaving K and this house. I think I would try to get my own place once I am able, and still be open to relationship with K if he is willing. It would still be a major change in relationship with him; we have been living together in very commited relationship for over 4 years now, and I love waking up to being in the same house with him, even if it's not always the same bed.
Does anybody see any hope here? It's hard for me to see any...
Feeling pretty broken hearted.
*I am currently waiting for a SSI Disability Hearing due to severe fibromyalgia. I have no money to contribute, and little energy, though I do housework as I can (in little increments of 5-10 minutes at a time). D feels that since I am not contributing more than that, I don't have a "partnership" with K, and that I have no right to be here.