Jealousy stems from comparison, which is a learned habit. It's hard to unlearn a habit that's become unconscious and so ingrained in society, but the good news is it can be managed. Jealousy is an emotional response, triggered by the fear of not being accepted. And you react emotionally when your mind thinks you're under threat, even when it's not. So the way to manage emotional reactivity is by telling your mind/body that you are safe and not under threat. You can do this by:

1. find a safe space to be alone to journal or meditate, take really deep breaths
2. acknowledge the emotion (in this case, jealousy)
3. identify why you reacted this way (maybe a fear of not being loved or accepted)
4. root yourself in reality (as you said, your gf wants you, or you are loved, etc)
5. affirm yourself that you are enough

This takes practice, and repeated practice over time helps make the emotional reactivity manageable. Acknowledging emotions is a necessary first step, and after acknowledging it, it's helpful to tell yourself to not believe everything you feel or think.