I love a girl but she loves someone else, how do i get her to be with me?

I love this girl, she is my ex and we dated for a few months but she broke up with me a little while ago. I love her and she loves me back, she told me she has a new boy friend and she loves him more than I, they have had the futures planned out even. How am I able to show her that I am better for her? I know this is selfish but she just means a lot to me. The guy has mistreated her many times and they still get back together, I have never mistreated her but she still dumped me and then goes out with him when he comes to her doorstep appologizing. Can someone help me convince her I am the one for her?