Just realised that I post I made yesterday isn't there .
FA/LWTG when there are problems in marriages, it is really hard to see God's purpose. All I can suggest is you try not to think about all the things that could happen, for example re: DH finding happiness with a fresh partner in your case, FA, and concentrate on day to day living, and practicalities. And of course, not forgetting prayer in all this. At times like this prayer can be really hard, especially if what you pray for is not in God's plan for you.
I prayed really hard for God to save my first marriage - because I believed that was what God wanted and because I couldn't imagine life without my ex-"D"H, and because of the Catholic church's (in particular) teaching on divorce... the alternative was unthinkable in my eyes. I imagined a celibate life on my own, with no children, or possibly an emotionally draining 10+ year battle to get an annulment (at which point it would probably have been too late to have children). At no stage in all my imaginings did I see myself where I am now!
I don't know how you both are going to discern God's will in all this. SOMEHOW you need to leave it up to him, see if you can hear him telling you what his plan is - ask him if there is some way he can show you what his plan is.
LWTG - is there any possibility of you getting away for a break, maybe a retreat?
Praying for you both, and for the families of those dear children.