I wouldn’t send it, neither of them are worth your energy.
As usual there are two dynamics at work, who is to blame for the cheating versus personal accountability for one’s own behaviour in any given situation.
He is solely to blame for betraying you, he chose to cheat. Not her.
She, however, is personally accountable for knowingly engaging in behaviour which she knew would cause immeasurable hurt and harm to another person. People who have been cheated on are incredibly angry with their partners, obviously, but also with OW.
Here’s where the MN bone of contention usually lies. I think it’s absolutely understandable and justified to feel anger towards the OW as well as your partner, but I would be angry for different reasons, I would not blame the OW for the betrayal. However, I would be angry and upset with anyone who deliberately engaged in behaviour which they knew would hurt me. It doesn’t matter to me one bit whether I am a stranger to them or they ‘owe’ me anything or not. To suggest it’s ‘wrong’ to be angry with those who do things which hurt us is ridiculous to me. Just because your motive for doing it was nothing to do with deliberately hurting the cheated on partner, doesn’t mean you’re unaware that what you’re doing absolutely will hurt them. There is a personal moral choice at this point, which has nothing to do with the cheating husband or partner. A personal choice about what we think is or isn’t an OK way to treat others. It’s not OK to engage in behaviour which hurts others, whether you care about them or not, whether what you do hurts somebody indirectly or not. It’s just not OK to do stuff which you know hurts others. I’ve never met anyone who thinks it is.
So, I completely understand your anger towards her, there’s a place for it alongside your anger towards the one who is responsible for betraying you.
So why not act on this? Here’s why I think it’s a bad idea:
We don’t usually feel indifferent to those who do things which hurt us.
However, as you’ve already heard from an OW, most of them who post on MN seem not to care. It’s up to them to search their consciences as to whether or not their behaviour towards another person was laudable or not. Usually OW prefer to wash their hands of their part in hurting you by referring you back to your partner, as he’s the only one participating in wrongdoing and somehow that absolves them. They act as if nothing they did could possibly have anything to do with you. It did: the man they were sleeping with was deceiving you and destined to hurt you by doing it, their sleeping with him would hurt you, yet they still joined in. Not their job to police his behaviour or prevent him cheating on you, no, absolutely not, but very much their job to police their own behaviour towards others. Not many people think it’s right to sleep with anyone in monogamous relationships, whether the spouse or partner cares or not, basically because they know somebody else will get hurt by their actions too if they do. Just because somebody else can’t police their morals doesn’t give another person a ‘bye’ on their poor behaviour to shrug their shoulders and join in.
Not looking too closely at our own behaviour and chucking it back on others whom we know are behaving badly, helps get rid of guilt and shame enormously, like a child saying “Well, he/ she did it first! He/ she was doing it too! It’s nothing to do with me!” absolves us of any personal accountability. It doesn’t.
So I reckon it’s a waste of your precious time and energy. If she’s got no conscience about it or is avoiding looking at her conscience, she’ll shrug it off.
It’s understandable to want to hit back, but really, really not worth it. Keep your dignity and moral high ground, that feels way better, believe me.
Look forward now and live your best life OP, write out whatever gets it out of your system and then destroy it and move on, they’re not worth the energy you’re investing in them and most certainly don’t merit it.