I deal with exactly the same thing as part of my depression - a belief that I made a mistake in the past that will prevent me from being happy ever again.
In fact, if you look on DF, you'll find a large number of posts from people who believe that past mistakes will prevent them from being happy.
If you read Eckhart Tolle's book "A New Earth" (which I heartily recommend), he talks about how the story of everybody who comes to him for guidance could be titled "Why I Can't Be Happy Now".
This is not to discount the pain you're in - I'm in it too.
It's more to say that we all make these "mistakes" - and the thing is, we have to learn how to deal with them - which involves accepting that the world is the way it is, and starting from there - rather than wishing something was different. Which is ridiculously painful. But it's the only way forward.
One other thought - cognitive behaviour therapy talks about challenging negative thinking patterns with more realistic, objective thoughts. You say lots of people haven't made mistakes as big as yours. You didn't **** anybody; you haven't committed any other crime, far as I know; you don't have kids you don't want; you aren't in so much debt that you can't possibly get out of it. Etc.
Again, I'm not trying to minimise your pain - the level of pain has nothing to do, in my experience, with the size of the "mistake". But it is good to keep in one part of your mind a sense of objectivity, as much as possible.