Hi. I am a freshman in highschool, and over the break, I found myself completely frustrated. I suddnely have trouble counting, and whenever I try to vibrato for a while my arm gets tired. Once, my left arm got so tired I couldn`t even play a scale. I also developed this tendency to bounce my bow so now I`m having trouble playing longer notes, slurring, and my arm always gets tired, unlike before. Does anyone have any suggestions or any excersizes to improve my "stamina",and help me stop bouncing my bow?