Friend, this woman is not worth pursuing, really. I know you love her very much, but her heart is not here at all. When a good woman is with her boyfriend, other men are just floating clouds. If she really loves you, she won't have any contact with her ex-boyfriend. This is a woman's duty.
Leaving her is a wise choice for you and her. Love a person doesn't have to be with her. Seriously, I am worried about you, because I have seen too many good people, especially good men, wasting their time for an unreliable woman and finally failing. Breaking up decisively, though realistic, is the right choice. Life is long, really.