Doubtless_Mouse: Why do you have to give reason at all. Isn't something along the lines of. "Tom, I think we should stop seeing each other." Tom says, "but Mary, why, I thought we were perfect for each other?" Mary replies, "Tom, you are a great guy and I think any woman would be lucky to have you, but I just don't think you are the one for me." "I am looking for my life partner, soul mate, etc, and I just don't think you are the one. I am sorry I enjoyed our time together but its time to move on."
Or even better yet, Mary says, "Tom, get the f&ck out." Tom says, "What...why...how come, " Mary replies, "Its over I know about the other woman." Tom says, "Mary you are the only woman in my life, I love you." Mary replies, "Tom I know, stop pretending, I have known for a while now. I saw you." Tom will eventually do one of two things, A) Admit to having an affair in which case Mary can easily break up with him or B) emphatically deny the accusation, in which case Mary can still break up with him because Tom thinks she believes he had an affair.
Maybe I should go into marriage counselling...