I need advice on how to deal with my current situation.

I got married in my early twenties and that marriage lasted two decades. We have one elementary school aged child together. One day without warning my wife came to me and said she wanted a divorce. She did not cite any particular event or reason other than our relationship dynamics and interactions were not what she wanted anymore. We had married young and she wanted something different now. She said she wasn't seeing anyone and didn't know when she would feel comfortable dating again but perhaps sometime in the future. I was shocked and couldn't imagine telling our child and how it would affect them, not to mention the embarrassment and sense of failure. We both grew up in conservative Christian homes where this sort of thing was considered deeply wrong.

Nevertheless we agreed we did not hate each other in any way and tried to make it the most amicable divorce possible. No big fights or legal disagreements. We worked everything out ourselves. Joint custody 50/50. We separated our lives but still saw each other and spoke multiple times a week. Friends and people who knew us commented on how mature and amicable our split was.

Then the ex told me she had met someone and they were dating and were soon going to introduce our child to this man. I was a bit taken back by the speed but tried to be OK with it.

Fast forward to a few months after our divorce was finalized and a woman contacts me out of the blue and tells me my ex-wife and her ex-husband were having an affair for *years* behind both of our backs. At first I was in disbelief. My ex was too good of a person to do such a thing. I asked for proof, for receipts. She provided plenty and then some. Overwhelming evidence that she had found during their divorce proceedings. They had wrecked not one, but two families. I had been lied to and deceived and betrayed to an extent I didn't realize was possible. My mind was reeling and I felt like my whole world had blown up. I didn't know what was real and what was fake in my marriage and life. Not only had she been lying to me for years during our marriage but even after we split up she continued the charade.

Now these two are still a couple, are moving in together and are engaged to get married. My child will be living (half the time) with the man who carried on an affair with my wife for years behind my back. The disrespect, anger and resentment I feel is overwhelming. I will be forced to be around this person at times whether I like or not. I want to have a good relationship with my ex for my child's sake, but this feels almost impossible. Does anyone have any advice to give?