OP I have also been through similar, albeit a very long time ago. Someone who walked away from my children without a word to them, came back 6 months later, then went again within a week. [Was asked to leave by myself as I realised was only with me because other woman wasn't interested].
We go through hell, the insomnia, the heartache, the betrayal...thinking it will never end, we will never be happy again, life is over. But the shock dies down, we get through a day at a time, cry when there is no one there. It changes. Things happen, we move on without realising it.
All I know is my sons saw the pair of them in town rowing in public. Some months later I saw them too, arguing. Do not think the pair of them are happy and living a wonderful life. They aren't. Too much in your post screams out that their 'relationship' will be disastrous, unpleasant and will implode. Your H has lost children and grandchildren and you. You will never respect or trust him again.
Your life will change although you can't believe that now. Living without a partner is difficult but living as a single woman can be liberating, free and fulfilling. You will get there. Keep getting support on here. I only wish this forum had been around when I went through a terrible time.
I wish you all the best, strength and much respect. Your family sounds great, what a great job you have done. Your H has been cruel, stupid and selfish, and he has thrown his life away.