I’m afraid how you’re feeling is very common from the experiences on this forum (and my own). For me it took nearly seven months of not eating or sleeping before I finally picked myself up. Trust me, it does get better, but you have to just keep going somehow. If you’re going through hell, keep going, as they say.
Your ex may not come back, but he’ll miss his kids at some time and things will settle down. But for now, it’s just one day at a time, and your kids need you.
All the best, PM me anytime.