"I was a real deek head. But that is still no excuse for an affair."
whether or not its an excuse is irrelevant and open to judgement. you can determine what is an excuse only for your behavior, she gets to decide whats a valid excuse for her behavior. it would be easy enough for her to say the same for your deek head behavior. you can go around and around forever.
the point is, you can only truly judge yourself and your actions.
and the reason for this is you do not know what she felt while you were being a deek. or how long she felt this way or how much it hurt her.
you can only apologize and deal with the consequences and take responsibility.
"In a ham and eggs breakfast, the hen is involved, but the pig is committed".