Feeling Invisible And Depressed?

Just to let you know im 16 an average high school teen like everyone else.For some reason ive been feeling sad,invisible and depressed for no reason idk why.Part of sadness comes from a girl who im best friends with who i like i think she is just using me even though she tells me she isnt,my invisibility comes from getting left out from getting invited to parties that my friends get invited to without me and few occurances of lack of attention from me,my majority depression which is sadness comes from i think no one likes me in this world and no one cares if im in this world or not and i keep thinking all of my friends use me instead of being friends with me.Plz i need adv and help im sitting in my room in the dark depressed feel like crying :(