Have you ever been with someone not over their ex?

Been in a relationship with someone who wasn't over their ex but they told you and you thought that they were? I'm really hurting at the moment, how do you get over the fact feelings might not have been real or genuine on their part? It's so painful and hurts so much😓

*I didn't know they weren't over their ex when we started dating I was led to believe he was, now I'm the one paying for it