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@Qzmp1, I appreciate the tough spot you are in. Addiction is a place where we do a lot of justification, and end up doing things we previously would have called evil, while telling ourselves it's not that bad. Then we escalate, and see it's not that much worse than the thing we already justified, and before we know it, we've crossed some hard boundaries and we know nobody will understand. Then we encounter some kind of breaking point, and are confronted with how much we could lose, and it's terrifying. Our compulsion to justify doesn't stop. We search for reasons, trying to convince ourselves we don't have to do the thing we know is right.
Consider; if she were doing what you have done, you would deserve to know. You said you had an open marriage previously, but not any more. You had rules that you agreed upon, then you agreed upon a different set of rules. You broke those rules. She deserves to know.
Also consider, she probably already knows something is wrong. I have been on this forum for years, I've seen hundreds of guys come through here, and I can personally attest, it is extremely rare for men to have a porn addiction and for their SO not to know something about it. Sometimes they know everything already. Sometimes they know he looks at a lot of porn, and she doesn't know what kind, neither does she want to know. Sometimes she knows he looks at porn, just not how much. Nearly every time, at bare minimum she knows something is very wrong with the relationship and she does not know what it is, but would like to help fix it. In some cases, she decides to end the relationship. In most, she at least tries to stay. It is extremely rare for an SO to have no clue, and be totally blindsided by an addict's disclosure. I would argue if she was, they didn't have a healthy relationship anyway. If she can't see the distress he's in, she's not paying attention.
Also consider, telling her what you have done is not what damages a relationship. Looking at porn and hooking up,
that is what damages the relationship. You are just now recognizing that fact, late. I am sorry, that is an extraordinarily uncomfortable place to be, but you can't undo what you have done. If your water pipes froze and burst, not telling anyone about it doesn't mean it didn't happen. You are going to have to admit you made a mistake and aggressively fix that problem, and get help to fix that problem, or you're going to have to deal with that problem AND more secondary problems in the future.
The question is not, in my opinion, if you should disclose, but how you should disclose and how much to tell her at a time. One of the things porn does to us that experts do not recognize, but the vast majority of recovered addicts and SOs acknowledge, is that it makes us insensitive, selfish, and bad at communication. This is not a good position to come from when handling such an important relationship problem, but at the same time, it needs to happen. It's like someone needs life saving surgery right now, but the surgeon is drunk. Doing the surgery, or disclosing to your partner right now, is probably going to cause some damage. At the same time, waiting until you sober up is not a great option either. I recommend trying to ease into the conversation, instead of dumping your sins on her like at a confessional. "Can we talk?" always signals a serious conversation, so she will know something big is coming and can choose to delay it if she is not in a good position. "I have been having a hard time with anxiety and some other problems" is a good step, but you have to follow that with "I have been trying to fix that on my own, but not in a healthy way, and I got in pretty deep." All of these give her the option to shut it down if she doesn't want to know details, or for her to ask for them as is her right. Then she is given her choice to decide what she wants to do.