Hi Renemorcom5,
Welcome to moomie!
I am so sorry to hear about your loss and difficulty TTC.
It seems as if you have checked out all the possible medical avenues of possible problems. Have you considered that there could be a deep subconscious block perhaps caused by the traumatic loss of one of your twins? Sometimes the trauma of what had happened is still trapped in your body and your brain could be protecting yourself from a future pregnancy due to fear that the same situation may happen again.
My hypnotherapist has told me of numerous stories of women who have battled with infertility and it turned out there was a deep subconscious emotional block and that after this was resolved these ladies became pregnant.
This is just what came to mind when I read your moving post.
Wishing you well. xx