My thought is that you are doing all you can. If you are supporting her in her process of working through their issues, and taking care of yourself and attending to your relationship with D... I don't see there's anything else *YOU* can do.
At this point, she really has to do some self-searching and figure out what she wants in life, and from this group. She may find that her wanting to be number one is something she can work through, and may be able to figure out why that need is so important and get it met some other way. Or, she may decide that she can't be in the type of relationship where she isn't number one, or loved the MOST (whatever the heck that means). Either way really she's the only one who can figure that out.
It's great, though, that you all are there for her, that D is being firm, but loving, and that she is getting support and love while she works through this.