My warbow is only 80 pounds, because I just haven't run across any heavier bows, yet. But I'd really love to have a 100 Pounder. Pulling the heavy bows is like lifting heavy weights....it's mostly technique. I would practice on lower weights and work up to the war weights. And, you might try a little progressive weight training as well, especially on your scapularis and traps. get upour technique dowqn well, before you move to the higher weights, to avoid injury.
Remember, the Old English Archers could pull 120 pound warbows, but their bodies were permanently deformed from the process. You don't want that. An archeologist can tell from the appearance of the skeleton of Dark to Middle Ages Englishemen if he was a longbowmen or not. The archers all have deformed bones from a lifetime of pulling heavy bows.
Go easy. Take your time, and you will get there.