1. Confess to him
Although this is not the best way to solve jealousy, it is the most direct way to protect your friendship and NSA relationship. Although many hookup finders are very resistant to this proposal, because it is too risky for them, and they are also shy to do it. But if you don't want to lose more because of this, you'd better do it. Jealousy is an obvious characteristic of you falling in love with your friends. When you tell your friends about your emotional changes, if they just like you, maybe your FWB relationship will become a romantic dating relationship. Of course, this is the best result. Even if he doesn't like you, you can stop suffering from this kind of jealousy by saying it.
2. Record your agreement in your mind
This suggestion is aimed at the adult affair finders whose thoughts are not completely occupied by jealousy. If they see their own FWB with others, jealousy will drive them crazy, it means that the conditions they set with their own FWB have no effect. So it's futile to keep their agreement in mind. However, when jealousy starts to encroach on your heart, this advice can help you to some extent. I believe that in every NSA relationship, both sides have set their expectations and rules, and will definitely follow them. You are a person who always strives for honesty, so you don't want to break the rules you set. Then, you should let these rules remind yourself repeatedly.
3. Keep yourself busy
One of the reasons for your jealousy may be that you have too much free time, so these boring things will crowd into your mind. When he and another girl are playing outside, you have nothing to do and let your imagination erode you. Why don't you keep yourself busy and use the time you spend imagining meaningless things to do meaningful things? You can read books, watch movies and dance. All of these are good ways to help you improve yourself.