Could You Write Something Like This?
Letter to the Mistress
I wish I were contacting you under different circumstances. Nonetheless, from one woman to another, I respectfully request that you back away from your relationship with Bruce to allow us the space we need to complete our divorce privately and peacefully. I have asked the same of Bruce and he has agreed. I think, if you were in my position, you'd greatly appreciate the opportunity to work through the divorce process without added distractions and stress.
After almost 34 years of marriage, Bruce & I have a huge history to sort through and a son whose feelings and opinion of his father's integrity and character need to be considered. It's my understanding that your husband is okay with the relationship you are having with Bruce prior to the finalization of your divorce. He's a generous man. My children and I, however, are struggling with this.
Please don't misunderstand me. I wish all the best for Bruce. He's a decent man and I'm glad he's found happiness with you in his life. My hope is that the both of you will be able to put your relationship on hold (as far as spending time together) until Bruce & I have been able to finalize our divorce and have been able to get some closure. It's important that Ryan see you both step back so Bruce can demonstrate respect and humility and prove his integrity and character by doing the respectful thing - ending the marriage before continuing his involvement in another relationship.
Bruce and I definitely need to proceed with our divorce and we both hope we can work through it quickly and amicably. Once the divorce is finalized, the two of you have my full blessing to begin seeing each other again. I wish nothing but the best for you both as a couple and I hope you will respect my request.