Sorry for what you're going through, Headisspinning! When my marriage ended and the ex had a GF, the kids would go for the weekend and come back full of talk about how nice they were, presents they'd got, places they'd been to. I felt like a cast-off mum, just the mid-week carer.
But I promise you it will get better. Your kids will have someone they can talk to when the wt loses its grip on them as seems to happen with so many young ones. Hang in there and be there when they need you.