Irish-
I may not have children, but I understand what it's like to have those thoughts, as most of us suffer from them. Believe me when I say that you're not a failure, you have a beautiful son to prove that and none of us are failures. We've made it this far in life, so we must be doing something right.
You said that you tell your son silently "I'm sorry", but what I think you need to do is say you're sorry to yourself and forgive yourself because you never asked to have OCD, you don't want it, and none of us do.
When you start your therapy you'll see what triggers these thoughts and emotions and you'll learn more things about yourself and how you can defeat these thoughts and compulsions.
Good luck and let us know how you do!