If it says delivered and not read, she might not have seen it. If you're not friends with her on Facebook, it probably got sorted into a spam or requests folder
Who is this woman? A work colleague of your partner? Why did you send her a message? If she’s been having an affair / still going on / in love with your partner, chances are she’d be willing to say whatever he tells her to and will believe him when he paints the picture of you as a psycho crazy chick, which given you’ve messaged her, falls right into he will say. Does your partner know you’ve discovered him cheating?
Maybe give it more time and give yourself some peace. I know it's hard but take some time out, try to distract yourself from it for now. Make sure you have the support you need for when shit hits the fan or even if it doesn't and you have to take action again. Have you left him yet? If there is no repairing to be done between you so no going back, is it not better to leave him first then tell her you know once you've gone or was it just a hunch? I've just reread your post and when you say I believe, you either have proof/facts or you don't. Which one is it?