I HATE it when my critters get sick or injured! I always feel like a Terrible Cat/Dog/Parrot/Whatever Mom! I totally know what that is like - you think "I am responsible for this [insert type of life-form here] and if something goes wrong, it is on my head!"
I am so sorry this happened. I do not have my own horse yet, but I know that no matter how many times I am told that a horse alone in a padded rubber room WILL find some way to injure itself or tear its skin...I am sure that I will feel ABSOLUTELY terrible when it happens anyway.
*I* know you are not a Terrible Horse Mom, because a Terrible Horse Mom would NOT have even KNOWN that her horse managed to put a hole in himself, and even if she did notice it, she'd say "Bah. That will heal up itself, I don't need to call a vet. Pooh." and when the hole got infected, the Terrible Horse Mom would be all over HorseForum trying to find some way to keep from calling the vet even then.
So, you are definitely not the Terrible Horse Mom, but my guess is, no matter how many people tell you this, you will think "This is true, but...this is MY horse...I should have [figured some way to keep him safe]".
So, I will just say this: big hugs. Your boy will get better, this will all be OK. You would not be feeling this way if you were not tremendously responsible, and you would not feel this way if you did not love your boy from the bottom of your heart. This pain is the price of that love, and, as dreadful as it is, it is probably a Good Thing. More hugs.