When I first started reading your post, I was so angry at your ex. Because you were so loving and kind to her even though she had gaslighted you and betrayed you. You remained patient and detached.. truly the type of person no one would ever want to leave. Very hard to do! And she read to me as a really selfish, narcissist.
But then I read about your new casual relationship and I started to smile. This is great. It makes it easier to detach. The new person is aware and not looking for anything serious. And your ex is getting a dose of what she can lose. You did not date to throw it in her face or intentionally make her jealous and that is why this is just great. You can also explore whether you really want to return to your old relationship.
A couple of years ago there was an online relationship guru. That gave this advice..."Remain kind, agree with everything, see someone else". I dont remember his name, only that he was this elderly straight talker that was married to a woman 20 years younger but his pdf was free. You are following his exact advice.
I think it is wonderful that you have managed to detach, and GAL yet still DB. I think your casual relationship will give your ex much to think about. And I am glad that it is providing you with a distraction. My guess, is you are going to become a lot more attractive to her now. Other women want you, therefore you are more valuable. (i dont agree with this outlook but I know there are many women out there that think like that)
Hope you continue to post. I am gonna go back and reread this thread. You are doing incredible!!