Have you ever spoken to a real-life person about this plan - someone you respect?
I think you'd find it helpful to see that person's body language, facial reactions and to hear their tone of voice when you tell them this story. Because in your threads, nothing anyone writes can dissuade you from what we can see objectively, is a disastrous set of decisions that is likely to hurt many people, including yourself.
In answer to your specific question on this thread, now that you've told your husband you want to end the marriage, I'd advise complete honesty about the OM.
I don't know whether you ever read or contribute to other threads or whether you just post them, but you might have seen the many threads from people whose partners have ended the relationship while pretending that no-one else is involved. It causes unnecessary hurt, confusion and bewilderment and also prolongs hope that the relationship can be rescued. Once people find out there is someone else involved, it often helps them to see their situation - and their spouse - much more clearly.
Your husband's love for you is possibly causing him to worry that you're depressed or having a breakdown - any of the myriad excuses that people who love and trust give for their partners' hurtful behaviour. With shock comes denial and for people to get over loss, they need to move on to the other stages of bereavement.
I'd advise telling one neutral person your plan before doing anything and do it face-to-face. If you're still resolute, then tell your husband the truth. I don't necessarily agree that it would be a bridge-burning moment in terms of his desire to stay in the relationship. I think if he loves you as a person and that love is deep enough not to want you to get hurt and make a terrible mess of your life, he - like the neutral friend I'm suggesting - might just get through to you about how insane this is.
But I think you need to see all this in person, to realise it.