Oh OP, the worst thing you have done is 'agree to stop talking about it'. Why? That clearly isn't working and that doesn't make it go away. It will leave you Stationary, which is where you are by the sounds of it.
You talk about it as long as is needed, not necessarily the Affair details over & over (unless you wish to) but more the way it has left you feeling and how much it has devastated your life. There is no time limit on this. You need to have an outlet and be able to express your pain & suffering to your OH. He needs to help you to heal if you're staying together. After all, he caused this destruction! If someone else hit you deeply, you would talk about it as long as you felt you needed to.
Why should he never hear of it again when you're in so much distress?
How can you work through it & reconcile if you never even talk about it?
You will make yourself ill. You both need to be in it together.
Please sit down and have an open honest talk with your OH about this and your feelings and see what kind of response you get. He should be empathetic and willing to reassure you and help you in any way he can, and do things to help relieve your stress and associated feelings.
If he says he doesn't want to talk about it, then I fear you as an individual will really struggle to feel happy & secure in this relationship and that may mean you need to reconsider your relationship. Perhaps consider a few sessions of Couple Counselling too so he can really understand how you feel and talk about ways he can help you?
I also hate the expression 'move on'. The facts of what he did will never, ever go away. You will always remember it and live with it and it will always be tainted. It will never be pure and the same as it was before. Sorry to be harsh. You can't change the facts and that is why so many relationships end from infidelity and it is ok to walk away. Being together 20 years and having Child doesn't mean you have to stay.
But if you choose to stay together, which is also ok and understandable, the aim is to work through the emotions and utter destruction (because that's what it is) together, to have the best relationship you can given what has happened.
Sending hugs OP. It's a rough rough ride and I wish society talked more about the devastation that cheating causes.