My boyfriend is really insecure what do i do?

weve been together for 3 years and it nvr use to bother him that people would always ask him how he got a girl like me and tell him im out of his league bc he use to be so confident but now hes a mess this is all he thinks about, its ruining our relationship he thinks im "going to find someone better" and hes made "guys look at me" i try to make him happy and boost his ego but its not helping latley and im afraid this is going to break us up forever