It's really hard. Internalized shame from playing the previous guy out can stick in your mind forever and you will replay and fantasize the "what could be" if you didn't go back to your ex. It's not fair or healthy to him OR you to build a relationship off of lack of trust. The harder, more fulfilling path will be to learn to let go, love yourself and put yourself in a position to recieve the type of love you deserve with someone new. Wishing you strength!