Welcome to mummysg! I hope that you find comfort in sharing your stories with all of us here, and the more that i am hoping that we can help you and act as your support group.
Regarding your situation, I can only imagine how it feels. I know that you have given your very best to raise your two kids and now, here they are, choosing their father over you. They might have seen something, or experienced something that delights them. But whatever it is, that will never be enough for your ex-husband to take your children away from you. For God's sake! Talk to both your ex and your kids and tell what else can be done to keep the same arrangement. I am hoping for an amicable arrangement. For now, avoid stressing yourself that much. gather as much strength as you can because there is a battle that awaits.
We are here for you! Good luck.