Why am I so shy...??

You're confidence is shaken because you had a few bad experiences, so you need to build it up again, maybe you could find a guy you already know and trust who will treat you with respect, and then spend some time with him as a friend, just to get you used to being around boys who aren't going to hurt you and have your best interest at heart. Then you're mind will have that experience as a memory and reference point, and it will counteract the bad memories of the other two guys, so you won't be as apprehensive about men in the future. It's called positive reinforcement, apparently when dealing with a specific situation, humans need at least 3 positive experiences for every 1 negative experience or else the mind will associate whatever that situation is with the negative memory of it. I guess this is why you feel shy or nervous about boys, but don't worry you're only 17 so get yourself a good guy and you'll come out of your shell soon enough.