feeling insecure within my relationship

me & my boyfriend have been together for 5 months. we both love each other very much, and i feel as though we are quite serious and may go on for months, even years to come.
theres one little thing though which makes me feel very insecure. and thats the thought that he'll cheat on me. i have absolutely no idea why i feel this way. i've expressed my concern about it, i've told him that i'm scared he's gonna leave me for someone else but he insists that he wont and said it just wouldn't make sense because i'm amazing, that he loves me and is never going to let me go.
the other day, we were at some club with some of our friends and he told me he'd be back in 5 minutes. i saw him walk over to some blonde girl he knew, and it made me really angry and jealous so i walked off and cried. when i see him hugging other girls, or smiling at other girls, or even saying hello to girls he knows, it makes me really paranoid and jealous. i don't think he even feels remotely the same way when i say hello to my boy mates. its the only thing within our relationship that upsets me. i waste a lot of my energy thinking and crying to myself about it, worried that he'll cheat on me or find someone better. as much as i try, i can't bring myself to get over it. i know that some of them are just his friends, or just random girls, but it makes me really upset. we've broken up for holidays from uni now and all i keep thinking about is who he is with, where he is and whether he is with other girls.
is this irrational behaviour? and does anyone else or has anyone else ever felt the same way and exactly how do you deal with it?