Disbelief as never in a million years would I have expected my once-loyal and caring girlfriend to commit adultery with a married man and ruin someone else's family. 

Strangely this was followed by shame.  I know now that this isn't correct, but at the time I couldn't stop myself from wondering if it was somehow my fault.  If you are having these thoughts, get rid of them.  You cannot be held accountable for something someone else did.

After these initial emotions, all the others came flooding in:  hurt, pain, anguish, sadness, etc.  Healing is a process so don't bottle up those feelings.  Feel sad for the moment if you have to and cry on a friend's shoulder.  I got through it all with the help of my friends so I highly suggest that anyone going through the fallout reaches out to the people that care for him/her.  It is amazing how much having other people help you can speed up the healing process.