If you're not yet ready to let go of your exgirlfriend, and there's still some fight left in you, I have good news.
Forging ahead you'll build a much better romance on a much stronger foundation now that you've both made mistakes and know that you'd rather be with each other than be alone.
And if your ex is still with her new boyfriend after a month's time?
There are some very radical (and subtle) moves you can make to get her back in your arms.
First of all, how long after your break up did she begin seeing this new boyfriend? This means she's in a rebound relationship, and that's the best of all scenarios for you right now. Because your ex went from one relationship straight to another, . Doing that will only make you the psycho jealous ex-boyfriend.
This is a HUGE factor when it comes to later on getting her back. She'll resent you for trying to destroy her new romance, especially since right now she and this new guy are in the honeymoon stages of their relationship.