My initial guess is that the draw length is too long, but it's just a guess. Try posting a picture of you at full draw when you feel comfortable and about ready to shoot (from the side with the string and arrow completely in the picture). We'll be able to help out a bunch from that one picture. Another picture from above looking down at the top of your head with all of the bow in it would help with form issues, also. Your muscles quickly become conditioned to shooting. Once you get everything set up right, and it may already be, you'll find that you will be able to go from 2 draws then exhausted to 30 draws before exhaustion in probably a couple weeks. In archery, practice is more about conditioned muscle response than about aiming and shooting.