threesomes and sexuality

renee 🌈✨
okay just to start off, i consider myself bi and highly lean towards girls to the point where i don't like men at all if i'm dating a girl. i try not to label myself but i openly call myself a lesbian sometimes too. my girlfriend on the other hand is practically right between both genders equally. 
a little while ago she asked me if i'd ever be interested in a threesome. my interest peaked until i found out who it was. i don't consider myself a jealous type and i try to get comfortable with my bby talking about past experiences but... it was the guy she lost her virginity to. 
that night i kind of panicked. am i inadequate? would she rather be with a guy? it got so bad to where i couldn't sleep. i asked her like "are you sure that you are happy with me? am i adequate?" i overthought and analyzed every interaction i had with her and ended up making her feel bad about asking. even though i had previously brought up the idea of a threesome. 
i just want to know other people's experiences with trying to add another person into the mix. and maybe get some feedback on how to deal with my jealousy because i always thought i was better than that and fully accepting of multiple gender attraction because it's something i experience.