Why cant I talk?? In school, I barely say a word..?

I contemplate whether i should say hello to someone in the corridor its that bad.. I feel like due to being so quiet people dont like me, they just think why doesnt she talk!? School is just horrible, its like im with a bunch of strangers every day.. Even the people i sit with, im so quiet.. I can talk to some of them, but I get really nervous, and then I just dont feel like talking cause I feel like nothing I say will be good enough, like no one would want to listen anyway? But seriously, this has to stop, i know if i want to get to know more people, I need to be more social and conversational. Its hard though. ): I can be very open and talkative with certain people, but at one hint of rejection im back to square one, although usually its all in my head. Help!! Im an 18 year old girl btw, going to uni next year, so really need to start talking more to people!