The best advice I can give is to think of her getting railed by her BF and how inferior you are compared to him.
Stop. Think. He's likely taller and/or better looking than you. He touches her face, shoulders, breasts, and crotch and all you get to touch is yourself. She's used up now. Even if she was with you, she'd long for the days of having him back because you are inferior. You cannot possibly compete with any of the BFs she's had before, and she can no longer pairbond anyway. She's not worthy of anyone now, but society will still give her whatever she wants (thx feminism), and that's far more than you can offer.
This didn't work for my most recent one because she was a virgin (unsure if she'd even had a BF before, never asked), but it has worked in the past with other girls for me.
Now if this doesn't work, or at best works for a day, you now realize that oneitis is a disease. It's like having a broken finger... you can just take painkillers all day to stop the pain, or you can go to the hospital and get it fixed. The solution I present above are the painkillers. They might work, but for how long? You need to get the surgery, and the surgery in this case is really just finding a new, virgin thot or complete isolation (if possible) from the girl.