I suppose you chasing her, but she dismissed as with a reaons of saying that she can't forget her ex?

Main reason being - you can't take over her ex's place because certain or several aspect you sort of 'lose' to him. There is no winning or losing - but it is how she subconsciously calulate. In short, here is an analogy...
If you drink a good wine.... any other wine you drink that has lower quality... you don't like it unless it taste better.
Natural reactions. But seriously speaking this is not really a problem. You shouldn't have express your feeling to her - because I doubt you even know how she feels about you - which makes this attempt a risky and dangerous business.

Persist or look else where for greener pasture. Of course, its easier said than done, but... nothing can work without the first step taken. Sometimes you have to see the picture, not just what you feel

Cheers