There's no substitute better than the real thing. To start,turn down the weight and practice drawing and holding without actually shooting. Draw,hold, let down. . . draw, hold, let down . . . draw, hold, let down . . . etc. Do 3-4 sets of 10. You will be surprised how quickly you can begin to incrementally crank up the weight. As you get comfortable with each increased weight every week or so, give it another 1/2 turn until you get to the weight you want to use and continue to practice with that weight so you don't lose what you developed.
To SUPPLIMENT that, add in bentover dumbell laterals and a standing version of the same exercise using cables or resistance bands. That, bentover rows and seated rows should have you reaching your goal pretty quickly.