That's another really tough break up realization.
Your kids might get a new step mother in the process.
But... You stay their real mother and this won't change ever.
Cultivating a new harmonious relationship with your kids is definitely something you need to work on and develop consciously.
Be there for them! Absolutely!
I agree, that having to share them with his new partner is a big challenge but think of their interest first with that one.
What do you think they need the most?
The moment important decisions need to be made, it is still to you they will turn for advice and guidance.