Is it just me?

Does anyone else feel insecure with their partner in comparison to other girls? My boyfriend reassures me that there’s no need for me to be, but I can’t help it sometimes. Tonight, he went out with his brothers (who are older than him) for his 18th, and even though I trust him, I always have some sort of stupid thought in my mind that he’ll be talking to a load of girls, or they will be encouraging him. I just can’t help it sometimes :/ how do I stop constantly looking at my phone when he’s out and getting paranoid for no reason? It makes me feel shit...