If the problem is holding the bow up,then you MAY need a shorter draw and/or less letoff.Also having a straight arm instead of a bent arm should help.This allows you to have more back tension and in return ,making it easier to hold.

I am speaking from experience.I am only 140# soaking wet and only pull 58# right now but I pull hard into the wall with 65% letoff and am holding steadier than ever before.

Excercise and shooting are all good but without proper form,it will still be hard to hold steady.