If your W has pride, don'tyou think she will immediately make her decision, if presented with anultimatum, and the decision will not be in your favor? Of course, she may regeret it later, but why send both of you through this?
A lot depends not on what is said but how it is said.
Suggestion: simply tell her (peacefully) how you feel about her, how you still love her and want to build the future together. Then tell her (also peacefully but firmly) that you can't take it any longer, that this was too much for you, that you will still remain her friend and be there if she needs your help, but you are moving out of her life.
This is rather an LRT than an ultimatum (but the same meaning), and you need to be ready to accept her reaction, but it has beter chances to really convey your messge instead of making her angry (who likes to be cornered?).
I know at leat one case when this this worked IMMEDIATELY, bringing the W back and out of the OM, making her realize how important her H is for her and how little the OM means. She did not feel "being in love" with her H at that time, being rather upset and hurt by him (for many reasons), but this was a life/death situtation, and she did not hesitate a minute.
Five years later she is deeply in love with H and knows this is forever.
