Hi, glad you found your way here, but sorry for the reason. IMHO, the quicker the LBH shows his WW that he has no intentions of waiting around while she sees where her A goes.....the quicker she'll ditch OM. And I said "show" b/c words mean nothing to her.
In a nutshell, you need to change the dynamics of this relationship. As of now, she is the one who wants out......maybe. She wants to play around and then decide, but pretty sure she won't pick you.....is that about right? You, the LBH, are wanting to save the M. That tends to place you in a position of pursuing her. So, change that dynamic and become the WAS, and let her have the role of LBS.
You don't have to get into an A, or run out to file for D. Just make her think you are dumping her instead of her dumping you. You know how, don't you? Of course, since you've said you fit the nice-guy type, it will take a lot of b@lls to dump her, won't it? It won't feel "right" to you. You won't want to do it.
So, I'm just saying what would work for you to take charge of this sitch b/c ATM, she's the one driving, isn't she? The one who is being pursued is the one who has the power in the relationship. These words may not set well with some people, but that's the shortcut version.
I have not seen a successful in-house separation. At best, they take on the role of "friend" and are contented to live as roommates and in limbo to the end of time, or until another man comes to rescue them from their miserable existence. Get the picture?
Do you have kids?
Has there ever been cheating on either side in the past? Any other marriages?
Get legal advice ASAP.
Please do not agree to be her BFF.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!