I'm so sorry! I know you are heartbroken.
The clinical side of me says this might have been an Rh factor foal? Over the years, we only had one mare that had babies like this & ONLY when she was bred to a particular stud. They produced an Rh negative baby every time, but we knew, after the first loss, to test & stop any nursing of the foal for a few days (replacing colostrum from another mare). After a very few foals, we just no longer bred him & her. When bred to another stud, she never produced an Rh negative.
I didn't think of this yesterday when I responded to your post. I'm surprised, tho, the clinic didn't consider it. But, I'm not a vet, and this is only my thots on this.
As for beating yourself up -- it's not possible to think of EVERYTHING EVERYTIME....Please just sit back & know you tried. None of us is perfect. If we were, erasers would have never been invented. You obviously were very concerned & put the foal in the hands of the experts. I'm so sorry it came out this way. :no: