I think Byz nailed it.

I also wonder what came first, your ADD or his acting as a parent.

I recently went through a very unfocused period in my life, when I could not remember a thing, starting things and dropping, unable to manage a simple thing like paying my bills on time, or dealing with returns of purchases that broke, or even getting car to the maintenance. There was a cause for this lack of focus, it was not physiological yet, but who knows where I would have landed if I had someone along who wanted to play a role of a big daddy.

Stop beating yourself up.
And argument that a couple is having for 3-4 years could use couples counseling. It's not all your fault, hon, takes 2 to tango.