i feel like i’m loosing my friends...

hey, i want some advice on how i should cope with my current situation. i’m in high school and i feel like my friends are leaving my out of certain stuff. i know they’re gonna find other people that make them happy but i always thought it would be more gradual instead of fast you know? basically, my two friends and i used to be very very close but towards the end of middle school we started fading. one of my two friends and i even got into a big fight. however, i learned that i need to be supportive towards her making new friends because they make her happy. Fast forward to high school and the three of us are still kinda close. but, we have this spirit day next week and the both of them made a plan to dress up with another group of people without me included. i grew kind of jealous and i feel very excluded. i don’t know how i should cope with this. the first time this happened i just ignored it to the point where it was ignoring my friend instead of the problem. Anything helps. thanks for reading ://