Could it be too much weight, if your tired after 3 doz arrows. It is the weight. As hard as it may seem, it takes time to work up in weight. First, shoot every other day, or rest two days. Shoot for form only, without a target. Sometimes targets force you to think about aiming, when you should be working on form as you build up to a heavier bow. Otherwise, your reinforcing bad habits.
To work out with a heavier bow, without an arrow, draw back 12", stop, then continue drawing to 18" stop/hold, then to full draw stop/hold. Reverse the process letting down. Repeat 3-5 times RH, then LH. If it is hard, then you are looking at too much weight, and you should repeat this exercise, no more than a few times a week. Increase it gradually. Be conscious of any pain in the shoulders, or elbows on off days,or after a day of rest. Rest is important, that is when the muscles gain strength. If you lift your arms above you shoulders, or rotate and feel pain, stop. You may experience rotator cuff impingement if you try to work thru the pain. Rest and don't shoot until, it feels better.