Help me solve this problem i need to get over this feeling?

ok so i have a boyfriend, we are 2 yrs apart (16 and 18) and im a virgin and hes not and it has been startin to bother me. we have been together for one year and i cant stop thinking "i am not the first girl u said i love you too i am not the first girl u kissed" he is my first boyfriend and everything is my first while im his 3rd gf and nothing is his first.i have not told him this i dont want to because i feel its stupid and then we have been thinking of marriage once we are older(dont comment on this plz) and i think that the only first thing i will be for him would be his first wife and his first babys mamma thats it. i dont know what to do, should i tell him or should i try not to think about it. he kisses me and i think that, and it sucks. what do u think i should do? is this a stupid feeling? i know one of the girls that he slept with (we arent friends) and when i see her or she talks to me i cant stop thinking and picturing it. how can i get rid of this feeling?