How 2 deal with school problem?

ok well ever since the beginning of school i was mad because i was retained in my grade and all my friends were seniors and i hid it from them..through out the whole year i've been getting mad at that and now its graduation time for them and i know im suppose to be happy for them but im not im mad as **** because i should be graduating too every time they tell me about there college or something i have something mean 2 say or i'll just act like idgaf and really i dont..because im mad :/ how do i deal with this problem?