The odds are against it, but if you can show her that she’ll be happier with you than the new boyfriend you’ll have a good chance. Being thoughtful will go a long way toward showing her that. Even if it seems hopeless, don’t give up.
If it appears she’s moved on, still send her a card you wrote just wishing her a great week. But don’t look as if you have any expectations. Your thoughtfulness might really impress her.