I can't think of anything to add to the wonderful post redatlanta made. It's just as heartbreaking to lose someone under these circumstances whether she is with you or not at the end. It sounds to me like you made some really tough and well-thought-out decisions, and I think it's human nature to second-guess after something terrible happens. As red points out, it could have been worse, and with more suffering, if you'd brought her home.
You will work this through, and I hope you avail yourself of all the support you can to help you along the way.
Hugs,