You believe he has something you don't and are jealous for two main reasons:
1) you're still emotionally invested in your ex and the memories of your relationship
2) you perceive that her new boyfriend satisfies her relationship needs better.
Here are some ideas:
1) Ignore them. Since you must actually pay attention to someone to be jealous, consider how you can extricate your ex and her new relationship from your life. Unfriend/unfollow them both on social media, don't discuss them with others, don't take your ex's phone calls if she's trying to be "just friends" with you, etc. Being strangers with them will allow you to simply not know the details.
2) Reduce your emotional investment in your ex. Delete the photos of you and her. Also, either throw away or put away in a sealed box all the memorabilia from your relationship (e.g., cards, gifts, letters). Out of sight, out of mind.
3) Do a realistic memory review on your ex and the relationship. When we’ve suffered heartbreak or are missing someone, there’s a tendency to skip over the bad memories. We don’t generally recall them as cold, untrustworthy, selfish, critical, callous, aloof, etc. You may be over-glamorizing your ex by conveniently forgetting all of those negative traits. Remember her as she actually was in the relationship with you before you broke up. Make a list of descriptors with specific examples. When you’re feeling jealous, look back over the list and be happy she is someone else’s problem.
4) Take a good look at your own self-esteem, and work on building your own happiness (whatever that means for you). If you concentrate on your own success, health, and happiness and being the most positive person you can be, you will attract others your way. Eventually, one of those others will be a new lady friend who will make you completely forget about your old girlfriend. Make peace that your ex is gone and the rest will fall into place for you personally.