Finding My Way:
First off, don't beat yourself up. These feelings are common, and you have every right to them. My ex walked away after 17 years together and 14 years of marriage with two children. What made it worse, she walked away with someone and they lived together immediately after the separation. It was an emotional roller coaster, even many months after. One day I would tell myself I wanted her to be happy, and the next I wanted to see her fail in every way, just so she could learn a lesson that she had walked out on someone who really cared for her. Like you, I also forgave cheating many times.
You say you shouldn't care. But I think you have every right to. Yes, there comes a point where you need to heal and move on, but we're all human, and you can't help caring. Do you still have feelings for him, even though you say you do not want him back?
One of the best things I did was to cut off all ties with the ex except when it came to the kids. You say you are still great friends. If he's certainly moved on and is happy about it, it's only going to cause you grief if you are staying connected to him. I tried to do the friend thing with my ex wife for a long time. I even helped her move out when the relationship between her and the person she lived with didn't work out. But this only ignited more feelings and frustration in the end. Perhaps you need to bring closure by removing yourself from any kind of relationship with him? Keep it strictly business. Only deal with him when it comes to the kids.
Life with Divorce: http://www.lifewithdivorce.com
A personal blog dedicated to sharing my experiences, lessons learned, and advice on navigating through life after separation and divorce.