Glad you've taken on board that it would be a Bad Thing to contact the OW - it won't help your cause and it won't change the situation.
Say she doesn't know about you, but say that your letter made her realise. and she decided she'd made a mistake so ditches your lying cheating scumbag H - then what? He comes back and begs forgiveness? What do you do then? Take him back and wait for him to do it again? Because he's very likely to do it again.
And if she already does know about you, then your letter won't affect her at all because she's already made that decision to be with someone who treats his pregnant wife like that - she won't change her mind now.
Anything you write to her will be used against you, however reasonable you make it. If she's as horrible as your H then they might even laugh over it. Don't give them that opportunity or power.
So - as others have said, write the letter but then burn it. It's cathartic to write down everything you want to say to her, but never actually saying it is a far better route.
If you can, keep even your feelings private from your H. Let him know you're disappointed in him, of course, and that you are hurt and upset and angry - but don't let him know how badly he has affected you if you can avoid it.
I'd say being a "blank face" to the OW is an advantage, actually - try and keep it that way for as long as you can manage!
Stay strong lovely lady - you're doing well so far. x