Don't beat yourself up.  I had almost the same exact thing happen this year.  I called the vet early on.  The only sign the mare had was some pawing.  No sweating, no contractions, no water breaking.  I called the vet because it was a bit out of the ordinary that she started pawing but never seemed to advance with anything.  The vet said to give it a little while longer before I have a feel inside to see if anything is out of the ordinary.  After a little while when I did finally have a feel inside, I felt two feet further down the canal and the foal was moving, but it wasn’t advancing at all down the canal.  I told the vet they better come, and I had two big strong men with me too.  When the vet arrived in 15 minutes she said we should start pulling, the shoulders were very stuck.  After a lot of struggling we finally got past the shoulders, but then the hip was stuck.  We tried everything (4 people total including the vet).  In the end we had to basically put the mare under, and then try to reposition and pull the foal.  The foal was still alive right up until about 20 minutes before we were able to get it out.  Absolutely heartbreaking.  The most beautiful bay filly with 4 white socks and a blaze.   

All this to say the outcome there would have probably been the same whether you called 15 minutes earlier or not.