I checked out the website one of the other mom's put the link up for, the Trisomy 18 Foundation, and have read through a lot of stories and seen a lot of pictures and cried some. The pain you are feeling is, I can only imagine, the worst anyone could go through.
I did notice in just about all the stories I read, many of the babies died within just a few days of birth, some a couple of months, and a lot were delivered just as your Benjamin. I cried thinking about how hard it must of been for the parents of these babies to deal with the heartache, and can only say I really think, not ever having been in your shoes, but in thinking about it, I would have made the same decision as you. I wouldn't have wanted my baby to suffer.
You are a very brave woman A., I admire you. Second guessing yourself now can't bring Benjamin back. Regret is very powerful and can sabotage an already very tender, emotional and hormonally imbalanced psyche. What you did was a selfless act, that you loved your baby so much you didn't want to see him live for a day in pain or discomfort. You followed your heart and I commend you for that. I think in time you will come to a place where you are at peace with your decision.
I wish you all the best and hope you know that you have entire website of women who will gladly give you a shoulder to cry on whenever you may need one!
Trust in your heart, you did what was best in your given situation.