Despite AWALT, girls are all random and all different. Some like you more than others. You'll have a different "relationship" with each of them. So fixing your problem with "this one girl" doesn't mean anything in the long term.
If you're in a LTR, then "trying to make it work" and "putting effort in" and "all relationships involve work and sacrifice" just means you are a variation of beta bucks. Even if you're not paying dollars, you're offering commitment and support by being a point of stability in her life and giving emotional support. And chances are she's giving you shit (or the sex sucks) and you want to improve your relationship.
Thing is... you can't change other people. Some things will help your LTR work better but ultimately you are not in control. She'll do what she does, and you have influence over that but you can't force her to feel something different to her actual feelings. And she'll act purely according to how she feels. That's never going to change.
The solution to your LTR woes is to fix yourself irrespective of her. Get some abundance mentality back, get more options with other women, flirt with more women, be more attractive, care less about your LTR and be less reactive to it. She's probably getting less sexual with you, respecting you less, while every year you're putting more and more effort into what's basically a sinking ship. You can't say a few magic RP words and have her sucking your dick like a trooper again. Doesn't work like that. It's YOUR life, you need to fix that. Make choices that will work no matter what she does. Be more attractive, meet more people, lift some heavy shit once in a while, flirt with more women.
Conversely... if you're trying to get with "this one girl"... if you've not fucked her on two separate occasions, she is nothing more than an option. Treat her as such. She might have boyfriend, she might hate men, she might hate you. I guarantee she has a whole load of stuff (good and bad) going on that you know nothing about. You can't take her seriously as a sexual option before having sex on two separate occasions. Until then - she's nothing more than an option. So if your question is basically "there is this one girl..." then the answer is "she doesn't matter, get more options".
If your question is "I find that every girl I chat up throws up when she sees me", then let's discuss your situation. If you have ongoing issues about how to apply TRP, what specific bits of TRP mean, then ask away. If you have a situation that has come up that you don't understand, then ask away. If you want to know the best way to proceed with a specific situation then ask away BUT... from the point of view of "I'm seeing three girls but a situation has come up that I don't quite know how to handle and I'm curious on the best way forward".
But don't ask us "how can I make things work with this one girl that I really want to get with". That's one-itis, it's trying to get control over one situation rather than having an abundance of options, it's putting too much effort and emphasis on one girl, and it's the perpetuation of the blue pill media story of the guy who makes a gigantic gesture to win the heart of his one super special snowflake unicorn rather than fixing himself.