Nope Not going to blame this on the thoughts of having an open marriage at one point.
Your H chose to do this, and chose to not tell you. Open marriages work because there is an open line of communication, and the other spouse is aware of what their partner is doing, and is communicated, and verified that all parties are ok with it. Anything less than that is cheating, and that is very different.
Do NOT allow yourself to take any of the blame for this. That will only put you at a disadvantage, and allow him to hurt you again and again.
What he did is wrong. If he was lonely he should have joined a bowling league, or softball team, not found some other woman to bang.
Can you work it out and heal your M? I don't know, what I do know is that there has to be complete and total honesty and he has to do some work to figure out why he has crappy boundaries, and fix that. Is it worth saving? Only you can answer that.
I do think you need to be brutally honest with yourself, and figure out why you would want to save your M, and what you feel would be the deal breakers.
I tell all new BS's that join that they need to do some very concrete things.
1. See an attorney, find out your rights, and empower yourself with knowledge.
2. Get a full STD panel. He has a relationship with a woman you don't know. He can promise he used protection, as they all do, but most likely hasn't, you know he lies, so don't believe him. Do NOT have unprotected sex with him until he is done with her, and proves he is clean.
3. Focus on the basics right now, make sure you eat, sleep, and do one nice thing for yourself every day. If you can't eat or sleep, then talk to your Dr about it. Starvation and sleep deprivation make us mentally dull, unable to control our emotions, and make bad decisions. Nothing wrong with a little pharmaceutical help to get through this already harrowing time for you, taking care of aging parents.
Do NOT own one bit guilt or doubt over this. He chose to do this, he made a decision to deceive you. That is all on him. All you asked for was honesty and he couldn't man up an do that. That was his choice.
Keep reading, keep posting.
(((and strength))))