It's ok to stick up for yourself to them. It's ok to explain how you'll always be Dad and nobody can replace you in Title, Function or amount of Love you have for them. I've been there with 2 ex wives, 4 kids. I've had to remind them of unhappy realities from time to time. If you're truthful they will see that with time, and they'll appreciate your being honest and honorable, even more so depending on the crazy scale number your ex falls into