What we have to realise here is that everyone deals with things differently, as someone else said, most blokes would react in this way, and to be honest, so would I, and im a woman. This woman has been with you for 8 years so there mustve been something there on her part for it to last so long, the fact that she has behaved in this way, is probably totally out of character and is difficult for you to take in.
My advice to you, is try and work out where it went wrong, was she feeling neglected, emotionally or physically? Thats normally why people stray...Your best off trying to talk to her and asking her why, meet up in a public place (if she will) and talk about where it went wrong, yes you made a mistake with losing the plot, but dont punish yourself for it, its what most people would do!!!! Good luck x