I don't want to upset you, but my thinking on this kind of thing is that she wouldn't be leaving to be alone - epecially in your case where you KNOW where an OM has been involved very recently. Therefore, I would say don't press her on it, just assume that is a factor.
Unfortunately you can't do anything about it except continue to prove that your changes are real and making yourself the better option over the OM.
If you let her go (I know - very hard to do) she might come back. Is the OM single? Do you know much about him?
ME/XW:47 S21, D19, S15, S14 M:21 T:26 W moved 6/10 I filed 7/10 D final 4/12 remarried 8/12 W wants to R 12/10 and 4/11 but I decline