I went through something similar. I had an ex talk about sex with his exes. He never compared me, but it completely fucked with me. I might have overlooked him saying things if it weren't for him talking about his exes in relation to us. For example, he talked about fucking me in so many positions on a couch he sold to his co-worker. The comment was pointless because it wasn't possible since he sold it but it implied he fucked his last gf on that couch in so many positions and showed that he still thought about his exes sexually. Possibly reliving sexual memories with them through me.

After that comment I told him to stop but the damage was done. It didn't matter that he told me he thought of me everyday because the likelihood of him not thinking of them was low considering he brought up sex he had from a decade ago. He tried minimizing, of course. "I didn't talk about them ad nauseum. I didn't go into explicit detail. I only thought of them for a split second." Then he tried blaming me, like it was my fault that I didn't tell him bragging about the sex you had with your exes was bad, when that's just common sense.

I wasted a lot of time and emotional energy on him. If you can't get over it, then you can't get over it. It's shitty and immature of her to compare you to her ex. You shouldn't be with someone who makes you feel like you're living in someone else's shadow. You need to leave her.