Need advice

I never thought I would be writing this. So I've been with my dh for 15 years straight no breaks or anything. We have been married for a little over a year. We have a house together and have been ttc. I found out last night that he cheated on me 4 years ago with this girl. I am so beyond devastated and heartbroken. I am literally sick to my stomach over it. I don't know what to do. He swore it was one time and realized he made a mistake right away and would never do anything like that again. Everything has been amazing between us the last few months. So my question is should I stay and work it out or walk away from 15 years together. I know a lot of you are gonna say it is my choice and I can only make that decision while others are gonna say leave him because that was always what I would say and now that it happened to  me I don't know if I should just walk away or work at it. I at first said I wanted to walk away but after talking to a friend they said it is 15 years together and it happened before we were engaged but they also understand why I don't know if I want to try.  I love him so much but am so angry with him right now. I don't know what to do and just really need advice.