Welcome Horizons,

for taking the plunge and the trust you extend to us. Not easy to be open about mistakes in your past and it speaks loudly of the guts you have.
I want to make is work - but I also want to know how I am supposed to stuff the feelings of betrayal from his hurtful words away and forget them? The affair was not a good thing to do - I understand that - but I really can't conceive a marriage where I am supposed to be a figurative punching bag every month or so and then slough it off.
You can't make it work for him. Never. Only he can make it work for himself.
Quick answer: You aren't supposed to stuff those feelings. Your best move is to set a boundary, then enforce it. Leave the conversation immediately when he starts this--you can't stop him from saying such things, but you don't have to stick around and listen to them.
Indeed. The way to deal with abuse is with boundaries.
I want to make is work.[... . ]
I do practice the steps from the box to the right. [... . ]
We are using the book "High Conflict Couples" which I highly recommend. It's great... . as long as he is practicing mindfulness but when he steps away from it (and I've been used to it) it's like falling further down a giant hole and takes me longer to get up each time.
Here lies the problem. Don't get me wrong - High Conflict Couple is a fantastic resource and you both working through it is a big factor of you getting along quite well. You were able to get marriage counseling to work - unusual - most pwBPD relationship struggle as that can be quite invalidating at first. On the validation side it is all good but... .
... . the boundary side of the picture is totally missing. Your relationship is typical for members that have focused on validation only. At one time they plateau out. The general emotional temperature in the relationship has come down a lot and is possibly healthier than in a lot of "normal" relationships. But there is still rage in the relationship while the pwBPD outside the relationship is able to regulate and manage sufficiently to not rage. Then it is simply a matter that it is too easy to rage at home. Insight unfortunately does not help and boundaries need to be set up and confidence in them holding must be won in the usual and painful way. No shortcuts there. Considering the abandonment fear angle with him you may want to review validation of abandonment and how to take a timeout before getting into boundaries.
Above were my key points. Here more food for thought:
- When dysregulated a pwBPD will instinctively choose words that are close to their pain but also words that are known to have a major impact. He is bringing up the affair because he wants to hurt you in that moment. Not being there usually is the best option. It limits damage on your side. It takes away the weapon on his side and forces him to learn to regulate. Staying around does stall his progress to a degree.
- He is staying. He is struggling with your past. But he is staying at this point in time. Do words speak louder than actions? What are you holding out for - perfection?
- Have you forgiven yourself? You made a mistake with the affair. You burdened him with the knowledge (boundaries?). He is at times in pain. Watching that is not easy as it reminds you of actions you regret. Still you need to move on. The fact that his words hurt you so much may be an indication that you yourself struggle with your past.
- While much pain was brought on by your actions (and his also) you have invested a lot of love and energy to make the situation better. He has benefited from it, his D has benefited from it and you are a positive factor in their life. Can you close the chapter from your side? Accept that he may well need much more or infinite time and it is not under your control and waiting for it only puts you on the edge?