I'm going to keep it simple. I came from a place similar, I had an emotional affair. Hubby and I both joined some lists of mono/poly to help and many many stories there are opening a marriage with affairs. There is no going from a marriage that had cheating to poly. There's just not. FIRST, you have to work on the cheating. Yeah, mention that you don't think you are someone that can stay with monogamy, but first work on being honest and reestablishing trust. THIS WILL TAKE A LOT OF TIME AND WORK!!
Then you can move from cheating, to non monogamy, to ethical non monogamy and then to poly if that is your goal. But trust me, I have seen way too many times people assuming all you have to do is come clean and then say "Hey I think I'm poly and that way no cheating!" and then jump right into poly and it does not work.
Later we hear these stories of how the marriage is over 'but now I am free to be who I truly am!' which ends up being double speak for 'working on my marriage and dealing with my mistakes is to harrrrrrrrd!'
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year