I feel extremely jealous of my new partner's ex.
Did you ever meet your partners ex?
No, just heard wonderful tales about him.
She's with you now that's all that matters. You should be happy you are with her and not feel jealous.
I know I'm going to lose her, she will leave me for him.
This jealousy is causing negativity in your mind and soon ,perhaps, in your relationship. Let go of this jealousy before it ruins things for you.