I'm a BIG fan of compromise and teamwork and love hearing about couples who fight fair together to ensure time invested is not love lost...but it is not fun fighting for a relationship by yourself! Before you keep trying until exhaustion make sure you have a teammate. If someone keeps reminding you that they can leave at any moment, then holding onto someone who openly wants out is desperation. DO NOT put yourself through it...two people committed means two people need to want to stay. Spending all of your time trying to convince someone of what was is not "ride or die" for a relationship; it is draining. It is ignoring giant red flags that advise that a person no longer wants to work on things...this invites the entertainment of "options" which then becomes disrespectful...and ALL comes full circle to basically no longer respecting the original commitment. If it's yours and REALLY yours, maaaan you better fight like hell to preserve it; not each other, but with each other. But if BOTH people don't want it...let it go!
You have a choice between begging the person with the red flag or waiting for the person who avoids the white flag.