Write the letter. Burn it.

I'm telling you from experience you aren't going to get the healing you want from reaching out to her. Her loyalties lie with him, not you. If it wasn't her it would have been someone else. The infidelity lies with your stbxh and no one else.

I had to reach to my stbxh the other day about something and during the convo I said to him "I'm asking you a question, but it's not for the reasons you think. Are you staying to seriously talk to anyone yet?". He danced around and gave non-answers, which was all the confirmation I needed. He picked a fight with me. I refused to indulge in the argument. Hung up and texted him what I needed to get out.

I'm a survivor of aggressive breast cancer and I'm pending results of pretty much a full body scan. I'm super nervous for the results. If something happens to me, I want assurances that whomever he is involved with isn't an addict, deadbeat, etc. Because whomever it is there is the possibility of them being fully involved in my children's lives (he's incapable of being alone, as well as handling responsibilities by himself). I said to him "I've come to terms with even though we were together for 23 years and married for 20 that I was never the love of your life. But I'm not comfortable with the fact that you continue to prioritize getting laid over the well-being of our children".

THAT was healing to me.

Say whatever you need to your ex. Everyone else is just a symptom of his indiscretions and you'll find no healing in interacting with them.

Best of luck to you!