Vanishing twin

Our 8 week 4 day US was quite disappointing. 2 weeks ago we heard two very normal heart beats and both babies measured normal. ( We implanted 2 normal embryos, complete with genetic testing.) With this last US one baby had not grown since previous US and there was no heart beat. Thankfully, the surviving one is healthy and growing. Just wondering who else has been in this scenario. At first I was rational about it, but with more time to think on it, of course I'm coming up with a million things I must have done wrong. I don't want to seem ungrateful for one healthy baby so far, but we definitely had it in our hearts and heads we were moving forward with twins.