There is no specific right thing to say to her to get her back just like that *snaps his fingers*
All you can do is talk with her, look her straight and deep in her eyes and be honest and tell her how things haven't been the same and that you miss her more than anything else in the world, and that she matters to you more than any thing else in the world and that you can't stop thinking about her. Mushy stuff like that ya know!
Don't go too overboard, or yeah you will sound too weak.. but just let her know how you feel and draw the line.... past that is for her to decide whether she wants to involve you in her future.
You also have to put yourself in her shoes, and try to think what she's thinking, etc.
Women are complicated so don't try any easy cheap trix and kix, cuz they won't work!
Trust me buddy.
Goodluck
Also, keep your head up in the meantime and continue hitting the iron hard in the gym and stay focused, don't let all this shit drag you down, or you'll be in more regret than you are now.