What is it like dating someone who isn't over their ex?

The guy im dating told me he still loves his ex of 10 years, but doesn't trust her. And he is still in pain because of her. So isn't where he wants to be mentally / emotionally, but he is hoping for a relationship with me.
They have been broken up for about 3 years now.
What is it like dating someone who isn't over their ex?
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