Free --
I don't know your story, but I'm sorry, if only on general principles, that your M has ended this way. I know it's not what you wanted when you two first got together, so on that basis alone, I feel for you.
About your dreams: you've been around MB long enough to know what I'm probably going to say, that old adage about "time and patience." Well, I'm not (except I just did). I'm going to suggest instead that your thoughts of OW, even in your dreams, are necessary, even healthy to your recovery, particularly now as your D approaches.
It's called "unfinished business," it's called "what if," it's called "finding fault," and probably another half-dozen labels. Has forgiveness been a part of the process for you? Until you can be at peace with the situation, with your H, and even with the OW, you won't have achieved resolution in these areas.
My Divorce Care meeting last night focused on the absolutely necessary component of forgiveness. I'm not there yet; I still have a lot of resentment and questions and anger and unresolved issues. But I know that I eventually want to be there, and I hope someday that I am.
You need to forgive and let go. Release H and OW and you ultimately release yourself. Forgiving is for YOU. It frees you. I'll bet, even subconsciously, you harbor still some resentments and anger. That's why you dream about OW. I'm certainly not a professional psychologist but I know a little about this process and its components. Forgive him, forgive her, forgive yourself -- let it all go.
End of sermon. I'm pulling for you...
Ammon