How can I not be jealous of my best friend?

You are young, middle school is like that. Be yourself and always be nice to everyone and people will reciprocate that back to you. Your friend may seem cool now but things change as you get older, so don't worry about it. Also don't worry about getting your period... its nothing but an annoyance. Cramps, bloating, and fatigue once every month... I could do without!