Do you not trust his committment to you or has there actually been a committment to make you feel secure in your relationship?

Have you had previous experiences where you have lost a BF to someone who was flirting?

Getting to the internal cause of your jealousy which is NOT THEIR FLIRTING, is the key to resolving it. It's about what is going on inside of you & being aware/mindful of your own thoughts that are making you feel this way when it happens.