Jealousy

Okay so my boyfriend and I have been together 9 months and he has a few friends that are girls that I get jealous of! But I have been open with him and he ensures me that I am the only girl he cares for that way, and he’s been friends with these girls for many years. I trust him completely but when he does hang with them I still get jealous sometimes, does anyone have advice for me to chill out and not get jealous. I have met the friends and he has not been sketchy about anything I know it’s just my own insecurities but how do I stop getting jealous!!? Help haha