I need some serious advice im so lost ?

ok so im 24 i have a 2 year old son have been in a relationship for about 9 years yes i know im young and thats a long time .anywho heres my problem i always felt our relationship ually was fine it was satisfying, we did random things at times to spice things up and recently my boyfriend mentioned he wanted to try a .now im not oposed yet im not 100% comfortable with the thought of me and a girl and my boyfriend ( i would never go guy guy girl), but because he brought it up i feel like he feels the spark is gone which i understand for the most part because weve been together at such a young age . how do i do this without having a meltdown? and by meltdown i kinda sorta feel like hell be attracted to this other girl and eventually i will lose him is there anyway to prepare for this or geez i dont know advice please :) and please dont judge thank u