Jealousy is a conscious choice
One thing that I've learned that helped me more than anything: you, and you alone, have to accept that jealousy is a conscious choice. We feel jealous because we feel like others actions are a personal attack on us, that they are trying to take away what is ours. You need to make two realisations here.
- no one wants to take anything from you, an overwhelming majority of people have the utmost respect for another person's relationship, despite what pop culture tries to push.
- you don't own your significant other, they are not "yours" to be taken away. You may know this consciously, but you also have to realise this emotionally. Your significant other is an individual, entirely separate from you and your desires.
When you strike up a conversation with a girl on the street because she's wearing a tee-shirt from a show you like, are you thinking about sleeping with her? Are you thinking about your girlfriend, and how she would never know? Of course you aren't, you're thinking about the tee shirt and how it's cool you have something in common with this stranger. So, why is your girlfriend held to different standards than you hold yourself to? Why is she more likely to think that way when you wouldn't?
My point is, when you internalize and accept these facts, that jealousy is a conscious choice, then all you have to do is practice not making that choice.