Hi Kim, AnotherStander offers great advice and so perhaps other vets have felt that support stood on it's own, for you.
That said, as a newbie who is on moderation, it will take time for your posts to be authorized and show up. Be patient and keep posting, even though we all understand the anxiety and urgency you feel, right now.
While you point to not fighting as though there were no real disagreements, your H obviously felt it was OK to become emotionally and physically attached to OW. While it may or may not be true in your mind, he likely felt that he was not getting what he wanted, from you.
While that sounds more important than it really is, don't read anything into it other than, it is how he felt / feels. Understand that you will not be in competition with OW, if you wish to save the M. Rather, through DB methods, you will become an even better person that only a fool would leave.
As AS mentioned, do what 180s that you believe will help you become a better person, that you feel are important for you, and that might save the M. Also, GAL... it is great to help with your PMA, even if you may not feel like it, right now. Do your best.
Finally, have you read "5 Love Lanuages". You can actually find a test for LL online with your favourite search engine. Take the test for yourself and also try to figure out what your H is. It is very possible that his "love tank" became empty and you don't know how to fill it up. Knowing how the both of you give and receive love can go a long way to helping you grow and work through this sitch.
Keep posting, even if there are no responses, as it can be beneficial to journal and others will chime in with support.