I am sorry about the BFN-they are so heartbreaking. My previous IVF and FET (oe) earlier this year were CP's and this last fresh cycle was a bfn, which was more devastating to me because at least before it felt like my body was "trying". I am now pushing for DE but DH is not on the same page with that (yet?) so I relate to the rift this IF process can cause. I also have a child and yes, I'm very grateful for her, too, but I really, really REALLY (desperately) want another child so I think it makes sense that you are grieving for this not working out this time--I just had two friends grill me on "can't you just be happy with your one, healthy child??" so I know that pressure of thinking you shouldn't want more, like it's being 'greedy' or something, but it's NOT that way at all-we all have a right to want to add to our family and just because we have challenges in achieving that doesn't mean we don't deserve it as much as the next person.
I have gone through that "what did I do to make this pg not work out?" but I have come to accept that it is not in our control, it sometimes just doesn't work. We can't beat ourselves up for what nature is in charge of--I'm so sorry you are going through this loss and I am sending you healing thoughts and a virtual hug. Take care, ajc