There might be a lot of reasons (or excuses) that she may say just to soften the blow, but a rejection is a rejection. No matter what the reasons they said, they still chose not to be with you. If they can't be with you now when there's nothing much going on, why would you think there's going to be something waiting for you in the distant future? Future faking won't do it. Trust me.
Instead of burdening and punishing yourself with "what's wrong with me" or "why am I not enough", why not try to stop ruminating over what THEY think and start focusing on YOU. Because frankly, you maybe in their thoughts for now because of the rejection, but they'll go on their merry lives while you continue to count reasons why you weren't given a chance.
The die has been cast. There's nothing you can do anymore. Trying to think that there's going to be a chance in the future with her is like trying to fit a square peg in a round hole. It's futile, and friggin' destructive.
So start focusing on YOU. Because YOU matter.
Go no contact with her and I mean zero contact. Surround yourself with supportive people, friends and family. Go on and start a hobby that you have forgotten in the midst of this predicament. Travel to places you've longed to visit. Go to the gym and workout. Have fun. Reconnect with old friends, go out to meet new people but don't EVER use them as temporary replacement fodder for her just because you feel shitty atm and need someone to fill that gaping void in your chest.
Go no contact. Don't think of future chances. Stop the waiting game and respect YOU. Because you are better than waiting around for someone to make up their mind or to spontaneously combust into being available.
It may take time, but if you do what you can, eventually you'll arrive there, man. Moving on and benign indifference. It's beautiful and sooo damn worth it.