This is rough but you can’t compete with a ghost, and that’s basically what her ex-husband is at this point. He’s not real anymore, he’s just this idealized version in her head that she can compare you to. And you’ll never win that game because the fantasy version of someone is always perfect.
You need to ask her directly if she still has feelings for him. Not in an accusing way, but in a “I care about you and I want to know where we stand” way. If she admits she’s still in love with him, you have your answer. As much as it hurts, you can’t make someone love you if their heart is somewhere else.
If she says she’s over him but her actions say otherwise, like she’s always talking about him, checking up on him, or comparing you to him, then she’s not being honest with herself or you. And that’s not fair to you. You deserve someone who’s fully present and actually available.
Sometimes people use new relationships to get over old ones and that’s not fair to the new person. You’re not a rebound or a placeholder. You’re someone who deserves to be chosen first, not settled for because the first choice isn’t available. Know your worth and don’t let yourself be someone’s second best.