Tell her if she wants to work on her marraige then she needs to STOP disrespecting you by having secret conversations with FORMER PARTNERS... that is VERY RUDE
She's pushing your boundaries both to test you and to explore other options if her marriage gets worse... she is looking for a lifeboat.. she does NOT want to leave your marriage ALONE.
Start looking for a VERY GOOD Family Therapist and INFORM her you are looking for one.
Do not press her to commit to any sessions yet, just tell her your marriage is the most important thing to you so you are doing very careful research first.
Go OUT ad BUY Divorce REMEDY and Divorce Busting, as well as Not Just Friends by Shirley Glass.. those three...
When you discuss this don't make it sound like you are begging :
"I asked her today if hse stil had contact with him" makes you sound weak and she will exploit that.
Just say
"You said you wanted to work on our marriage, but you are contacting other men in secret... If you want to END our MARRIAGE you should at least be doing that to my FACE, not behind my back. I am working to improve our marriage, but I will not do that while my partner is engaged in infidelity behind my back. I see no point in me doing work on our marriage while you are in contact with him...you're just trying to distract me... I'm not an idiot, please don't treat me like one. If you want to work on a marriage, then put time into THAT, not into an affair."
Something forceful that gets the point across that you aren't gonig to be walked on...
You need to SHOW her significant improvement FAST.
Buy those books, make sure you leave them laying around so she might see them... do NOT WAVE THEM AT HER.. she's not an idiot.