Léon H

I am sorry that you are going through all of this, but really unless you take the chance to contact her, you will never get a resolution, and attempting another possible fiance or spouse visa in the future this petition will need to get rectified. 
 
I know it isn't what you want to hear, but biting on the proverbial bullet, should be to explain and let her know you understand, and let her know you forgive her, and then explain the situation as calmly and as nicely as possible is really your only alternative.
 
Good luck in your future. 

I did already, I had My doubt and when i face her with the facts she told me the truth that she's with someone, anyway she said many things that make me sure i wont continue with her..not only the cheating part……. 

You got various responses on this thread- some were unfortunately from users who gave inaccurate info and now you are back asking (some of ) the same users for more info...
 
As you were already advised the 129 process is done by the USC so it needs to be withdrawn by the USC. As the intending immigrant you can not withdraw the 129f. Do not go to any interview, do not give the Embassy your passport. The Embassy will not issue the K visa with out you showing up for an interview and presenting your passport- so there is absolutely no way the process will continue and a visa issued with out your participation.
 
You say you sent a letter saying the relationship is over. That is ALL YOU CAN DO. The Embassy wont officially mark the case as withdrawn unless the USC requests to do so- however the process will end because you are not pursuing it.
 
This can be an issue if you ever attempt another K visa. The embassy will require the previous K process to be withdrawn. Many people find themselves having to contact their old USC partner years later to ask them to withdraw it. I understand your situation and fear of her doing something harmful- but there is no other way for it be withdrawn.  

If it went to the embassy you need to withdraw the letter. We got married and had to withdraw our Fiance Visa with the embassy, for us it was a letter signed by a Notary Public.

Whoops my bad. 
 
I'd just send wherever it is a letter of withdraw. They're not going to ignore a letter telling them the relationship has dissolved. 

This has nothing to do with the OP. A beneficiary cannot withdraw a petition and the case is already at the embassy so there is nothing that related to USCIS at this level.
 
 
to OP:
you are the beneficiary, so you cannot withdraw a petition. Also the petition itself is already approved so there is nothing to withdraw. 
 
You can let the embassy know with your case number and an explanation that you are not together anymore and you will not pursue this visa application.

Sorry to hear this, but good luck to you in the future

Sorry it didn't work out for you two.

Sorry my friend, heal your hurt and move on, better to know before marriage

Agree pull the plug.

you can not withdraw the application.
 
since you are not the petitioner, the only one that can withdraw or end the petition is the USC,
 
I am sorry, and any distance in a relation can be strained.
 
I met my now husband in 2015, we met in person in 2016 got married in 2017, his visa was finally issued yesterday after a grueling 17 months from start to finish.
 
heart must be in it, and we are a 7-8 hour time difference but we make time for each other, lack of sleep, staying up, patience on both ends.
 
 
 
good luck in your future.

Yeah, it been a while that she's going on and off, one day she want me and she want to go back to France, and the next day she wokeup crazy on me and start fighting accusing me for false things, and ask for break up, she told me before that she want someone else and i give it time many times, but she keep going crazy each time ...i gave her everything i could give and i tried everything i can do so we become toghether but she Don't even try to understand its not my fault for being (Syrian) ! she even called me names few days ago because I'm Syrian like if it's a mistake or bad thing to be a Syrian, she was never like that, we was toghether in the same house for 2 years and she wasn't like that, she became crazy with me, and it's all related to many things, one of it is her willing to be with someone with easier situation, but probably she felt love, or she felt sorry about moving on …. I do love her and i will always but we reach no back point, i gaved her too many chances before ….but now im so tired and suffering..

Yeah, it been a while that she's going on and off, one day she want me and she want to go back to France, and the next day she wokeup crazy on me and start fighting accusing me for false things, and ask for break up, she told me before that she want someone else and i give it time many times, but she keep going crazy each time ...i gave her everything i could give and i tried everything i can do so we become toghether but she Don't even try to understand its not my fault for being (Syrian) ! she even called me names few days ago because I'm Syrian like if it's a mistake or bad thing to be a Syrian, she was never like that, we was toghether in the same house for 2 years and she wasn't like that, she became crazy with me, and it's all related to many things, one of it is her willing to be with someone with easier situation, but probably she felt love, or she felt sorry about moving on …. I do love her and i will always but we reach no back point, i gaved her too many chances before ….but now im so tired and suffering..

Thank you ...

Well friend I fell for you I do. Sounds like the visa process is the least of your worries. Regroup and move on. 
 
Work on getting your French citizenship, that's something to look forward to.  

I hate to bring you bad news, but its highly unlikely you will get a K1 visa. Waivers are restricted to immediate relatives, and to the best of my knowledge, a fiancée is not considered an immediate relative. I have heard of multiple K1s being denied with no permission to apply for waiver. If anyone reading this has a K1 approved after being told waiver consideration, please correct me.
 
The only waiver approvals I have come across are IR waivers (spouse, children) and some other exceptions like P3 or some weird visa like that. Also, these waivers are extremely rare, and are highly opaque in terms of what qualifies and who is actually getting them. 
 
If you search under my handle, I have posted a few statistics on this topic that might get you more insight into what is going on. 
 
Things are still moving on this front though, and next few weeks should bring more clarity into waivers. 

Waiver will allow you to enter the US despite your inadmissability.
 
Something the Consulate will handle.

Someody the other day was trying to work out how long, too many variables, impossible to say. I agree it is likely to be longer whatever that adds up to.