Hmm, well I have two things to say:

1) In general, trying actively to get her back will do no good, you don't want her to feel pressured by you.

2) However, don't let her get away with the excuse of "I think you need to date someone else for your own good" cuz it has been tried on me and its complete bullshit. Just make sure she knows that you want no one but her, and try to get rid of any insecurities she has about you wanting a younger girl. This could just be a test to see if you really want to stay with her or not, or it could just be that she thinks you will eventually regret not being with a younger girl and is trying to end it before that happens.