Confidence at a MLB/College camp

hey, im 14 yrs. old in our high school’s varsity team in oakland,Ca.
coaches tell me i have good potential, and that i’m capable of improving. im a pitcher, 3rd baseman & and 1st baseman.
my high school coach tells me that at the end of our season we are going to a baseball camp for the oakland a’s.
somehow at times i feel confident and know i can do good.
but at times i get negative with past performances and start to think negativly and get a bit nervous. :shock:
i would like to know how do i overcome my lack of confidence at a baseball camp.
Oh and to add to this. my brother really helps me in terms of doing better.
he attended to the university of miami and that is the exact university i want to go to to play college ball. he says that he is goin to take me out there to miami to their summer camps. same i get confident but think negativly on past performances and start to lack on confidence.
i guess im a nervous person like my mom. :?
i think all i need to master my mental side of pitching is to trust myself and my arm that i can do it.
if u help me on this one.
i will definatly take advantage.
thank you