You have a right to be going crazy. Either you can deal with it or you can't. Knowing your wife is having an affair is torture.
Read Smileysperson's thread. You can get your dignity back and take care of yourself and rediscover your own worth WHILE she is doing whatever it is she is doing.
But, you always have the right to say enough and tell her whatever you want to tell her. It will probably do NOTHING to compel her to change course but you need to do what you need to do.
Best thing is to read these threads. Go to the infidelity thread.
No one can fix it for you (I'm still trying for that). You are going to have to buck up and get really committed to taking care of you.