We were together for 2 years and we broke it up because I didn't feel anything for that girl, and our relationship was soooo frigid and I was with her because I feel a pity for her..
And now me and her best friend fall in each other and we talk and everything between us was very positive and we have a lot in common and I can fell something to her, something that I never felt it with the other. But still the point that she can't hurt her best friend so she decide to leave.
What can I do?

Nothing. Her friendship is important to her. Leave her alone.

Im with homegirl on this one. Stay away. It might hurt but damn dude. You shouldn't have put her in that situation.