This is where I get harsh.
This might be hard to hear.
But I mean well.
If a girl has feelings for another guy and you have to ask others what to do... it means you aren't enough for her.
It means you cannot handle her.
You see, even if someone gave you stellar advice or an effective gambit... it'd mean you get to keep her by a cheat. Which in itself is fine, weren't it for the fact that you'll run into similar problems in the future.
You'll start pulling harder, She'll start rethinking the relationship even more.
It's going to hurt your butt.
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You aren't enough for her, yet. So tell her this.
"I can't handle you. Yet."
It's a powerful thing to say. It's flattering, honest, but most of all: It says you aren't the weak type. You're a guy with prospect.
Such a comment by itself will ensure that 'a part of her' has you on it.
(Don't go into explanations outside of "I'd like to grow some more."
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Start doing what you did before meeting her. Go out and meet other women.
The idea of 'you' will start festering inside of her. She'll remember you differently. Adding grand ideas of how great you used to be.
You'll be wearing capes in her daydreams.
This new guy might be fucking her... but you'll be fucking her in the head. Lovingly, of course
Anyway, that's what I did.
I've seen it been done a couple of times (less intentionally) by others.
It's the best advice I can give you.