Pat, Sandi gives great advice and has done so again.
Look at it this way:
1) Setting up boundaries that define a viable marriage, maintaining your self-esteem, allowing healthy respect between both spouses - file this under the "can be controlled by patpat category".
2) Trying to assume what your W will think of it, how she will react, whether she will accept those boundaries or even care, how YOU will react to how she reacts, what impacts it might have, the fear it generates - same file, same category.
3) How W actually reacts, and choices she may or may not make that may or may not help your M - file this under "beyond patpat's control".
You're doing a fair bit of mind-reading and giving in to fear and letting 3) override the former two.
And as often pointed out here, work on what you can control, own your choices and actions, let the WAS make his/her own choices and live with the consequences, and leave the rest up to God.
What do you think you're having trouble with letting go of exactly? Your love for W? The part of you that may view her as a possession? The fear of losing her? The fear of confrontation? The responsibility you may feel for saving the M through your efforts alone? The fear of losing the contact you are having now? The fear of her reaction and making the whole situation "uglier"? All of the above? Hope you see I'm not being pedantic here, but if you can better understand what your fears are, it may help you better deal with them.
Cheers.
Me 42 W 39 Married: 11 Jan 1998, T: Since 1992 First Bomb: Sep 2007 Confirmed A/OM: 4 Nov 2007 Kids: D10, S5 Reconciled and together again after (alot of) time and heartbreak. 3rd kid, S, born 2 Jan 2010.