It is tough alright- no point in thinking what she is doing with this guy as whatever will run through your mind- will not make you happy! At the end you will just end up hurting yourself more and more.
What they do now- is not your concern-- she is a different person now.
Instead start to channel your thoughts on what the heck your going to do today and tom.
At least this way you will be a bit more constructive with yourself- even if it is for an hour or so.