i think it might also be important to remember that the memories you have from that time are all from the point of view of a child who was 7 and couldn't possibly have understood everything and known what all was going on, and since that time all the information you have is from the point of view of someone else.
My blokes kids have some pretty skewed ideas of things that happened between him and his ex, most of their memories are based on what they have been told by their mum, and its mostly a load of crap. I'm not saying that this is also your case, but maybe there were things that you didn't know about eg. parents not getting on before etc. Doesn't make an affair morally right but people sometimes do stupid things when they are not happy where they are. Just a thought :)
Also, it sucks to be the Dads new missus, trust me.