Well, it's easy enough to say "I love you"....but if she doesn't love you enough to ditch the other guy, she doesn't really love you at all.
I know it's hard, but having false hope might cause you more agony in the long run than making a clean break.
If I were you, I'd tell her she needs to make up her mind.....either it's you she wants, or this other guy. She can't have both.
If she can't make up her mind....a clean break might be the best way to go. It'll hurt like hell for a little while, but that will fade, believe me.
Good luck and take care.
