I am a wife and I have recently gone through the scare of my life. My husband wasn't sure if he loved me anymore and wasn't sure what he wanted. If this is truley how you feel, and know for sure that you no longer want to be with her and are not in love with her then, As a wife, and my recent experience, I think you need to tell your wife immediately. She deserves to know. Stop wasting her time, she deserves to be happy as much as you do and all this is doing is delaying the hurt and pain that you both are going to go through eventually. More for her because you have already been dealing with these feelings and it sounds like this is going to come as a shock to her. You probably don't want to hurt her, and hate the thought of all the tension and fighting and crying that is about to come. But it has to be done. Hey, maybe you will find out that she feels the same way about you. Who knows what the outcome will be, but you have to be truthfull and honest about your feelings, not only is it not fair to her but it is definitely not healthy to be living the way you are now and dealing with these feelings. If you are not sure, then I recommend you go to a councelor and discuss how you feel. Maybe she can help you sort things out before you say something you will regret....Good luck and I will be waiting to hear how things went.
p.s. It isn't going to be easy, but you both need to be happy. Life is too short.