How to be OK with being cheated on

My husband of 8 years has admitted that he has been cheating on me after I confronted him about an on-going pattern of not being able to contact him at work when he said he would be there. We had agreed before/on/after our vows that we would always have a monogamous relationship. I know I am only feeling pain from this because of what I’m thinking about it … that there is no such thing as a painful relationship… only my thoughts about it are painful. How can I reach a point of neutral thinking about this? Am I supposed to accept that this was always going to happen … it happened as it was “supposed to” … i.e., I was never going to be in a life-long marriage with him and so now I just move to Plan B? How do I move to Plan B and maintain my ability to openly trust another husband?