Yes, I am saying handle her differently to the way you have been. Because the way you have been handling her lead to her essentially not trusting in you to provide the emotional support for a fourth child. Let's face it, with the spacing you had, she was only going to have 1 more anyway, worst case 2 before the menopause.
Why did she get the IUD fitted? Deep down what were HER reasons.
Moreover, I doubt you are as in tune with what she is doing and thinking as you'd like to think, or you would not be in this pickle in the first place. Women are famous for being two faced and cold about these things. They can compartmentalise very well and then switch from night to day. So she might not be cheating with a flesh and blood person but do you really think she has gone without any relief for the last 18 months? You are a 40 year old guy and know how the world turns.
Why would she? She has an IUD she is hardly going to worry about masturbation and you are not with her 24x7.
And finally Gooch, if she's not going to leave you then why post in the first place? Stop kidding yourself. When a woman throws the word divorce around it means she's thought about it.
Best of luck, I have a weekend of stuff to do.