I need help =[ im having friendship problems, please help?

ok well my best friend and i have the most amazing friendship, we can tell each other everything and we cry together and laugh together and do everything together, shes older then me (shes supposed to be a freshman but got held back) yea anyway, so shes older then me and she was really ready for a steady relationship and she always complained and stuff about how no one would like her and im almost a yr younger then her and i got my period and boyfriend before she got her first so in a way i felt more mature and older and i was like a mother to her. so now she has a 'boyfriend' and its just soooo different for me. idk maybe its me that needs the help, maybe im the one who is making this a huge deal but ive known my bff forever and 100x more then anyone else and its almost like my little baby is growing up! i feel like im having trouble dealing with that because i feel like shes growing up so fast. one day shes boyfriend-less and now shes got a perfect guy whos sweet and protective and i feel like she is growing up too fast. everyone loves my best friend because shes so nice and everything and so EVERYONE wants to be her best friend, but it makes me jelous because i just feel like shouting "SHES MY BEST FRIEND!!!" and so i get reeally possessive over her and i logged into her facebook and there was a fb chat with her boyfriend and i read it (WOOPS!) and she told him that im a good friend and im funny and she likes when i laff and im fun to be around, but then she said that i treat her like shes my *****! well i guess in a way i do since im so possesive over her but i think im possesive because i feel like someday shes just going to forget about me someday and just like go off with other ppl and forget i was there for her the whole time. maybe i just need to learn how to trust her? can u give me advice? sometimes i feel like breaking into tears and being alone because i love her and shes like my sister and things are changing so fast and i just want us to remain best friends. =[